I gutted & rebuilt our '77
Surfside a few years ago, save for the few niggling details I can't seem to get done yet

Nice find if it's already gutted, that will have saved you some $ for the build!
A few tips from my experience. I see you're in Winnipeg, so will start with some geo-specific ones.
Online purchasing is your friend. If you put in the time, you'll find the deals. I ended up timing my purchases from US sources and shipped them all to Mike's Parcel Pickup in Pembina ND so with a weekend in Grand Forks I could make my pickup, skip the duty and save a ton on shipping.
There are a few places to pick things up locally, but I've always found things are a bit pricier, except for Princess Auto if they happen to have what you need.
Definitely rewire with 12v LEDs, including an external one & all internal illumination. I wired 3 120v circuits, all with GFI outlets, including an external one. Face it, based on the size of these, camping means a lot of outdoor time! I also incorporated USB charging ports and 12V cig lighter ports throughout. The latter are ugly and cumbersome, but it's a standard that just won't go away so you'll always have an adapter for it. You'll be rewiring everything, so may as well put in the conveniences, and make sure there's a
light switch near every bunk. I also recommend wiring to the roof vent for a fan and overhead
light. I don't have one, but you won't have to look too long to find the roof fan everyone either swears by or wishes they'd bought, especially without A/C.
I wired in a 30Amp
electrical system with power converter to run all the 12v things with built-in fuses, then added an inverter alongside it with an automatic transfer switch. Honestly it's a bit of overkill for these, but when shore power is available, I can run the air conditioner and
microwave plus any extras without a second thought. Yes, I put in both
You'll likely end up rebuilding the traditional layout, or close to it, but there are a few places you can push the dimensions a bit. The original layouts all had a floor-to-ceiling support somewhere midship: closet right behind the door, and the storage/appliance stack across from it. This provides extra roof support if there's snow load or anything like that to deal with. Not sure what the
weight limits are, but I kept basically the same floor-to-ceiling support locations in my rebuild because it seemed wise to do so.
Not sure from the photos if the hole is still there where the heater was. You'll want to patch that up with fibreglass - I managed to figure out how to tackle that on my own, despite my wife not loving the primer on the outside (at least I used a yellow primer) since we haven't redone the exterior
paint yet. Don't replace the heater with anything that size, there are far more efficient and smaller options available now, and that was a ton of wasted space.
With the heater gone, the first thing to do is upsize the
refrigerator. Nobody ever complained that they had too much
fridge space in a camper, though based on my own experience, I assume there are a lot of complaints when wives remove the beer to make room for dumb things like butter or eggs. Seriously, if you get nothing else from this, upsize the
fridge from the original spec. Once you start the research, you'll find the claim that the chest or drawer-style refrigerators are more efficient. This is technically true, though not practically true. The theory is that when you open the door of a conventional
fridge, the cold air "falls out" and the warm air now needs to be chilled by the unit once you close the door. Remembering from way back in school that warm air rises, you nod, because is is logical: the cold air "sinks" out of the fridge when the door is open. Practically however, your fridge is stuffed full of beer and butter and eggs now (all still cold), so there's not as much airspace in there as it would take to make much difference, because the tradeoff is with a chest-style cooler/fridge, you're looking down on everything, and shuffling it all around to find what you want instead of using the shelving in a proper fridge, and that's going to annoy your wife enough that she won't put your beer back into it once she's retrieved the eggs.
As to the plumbing system, I redid everything with pex, and put in a loop running outside to the front of the trailer where I can hook in a
propane on-demand water heater which was like $130 or something. They're
sold as portable but have mounting hardware. I installed a small bar-type sink that mounts below the counter rather than dropping in from the top. this means when I drop the cutout into the space, it rests on the lip of the sink and leaves a beautifully flush countertop space. Spring for a faucet with a built-in hose. I also plumbed in taps on the outside of the trailer (they sit inside a little door, buy the whole unit online) because again, so much of camping is outdoors.
Last tip. The first thing you're going to do after you've planned the build is to take out all the
windows. Don't worry, it's not as bad as it sounds, and you want to replace all the wood from the inside. Take the
jalousie windows inside and disassemble them so you can clean and replace any parts that need it - they're notorious for stripping so they won't tighten, but you cand replacement handles online. (keep track of left/right) Don't discard the wooden frames from the interior, you're going to use them to build replacements. Here comes the genius part. The
Surfside (at least most, I believe) did not have overhead cabinets above the rear bunk/dinette seats, but there was sometimes a shelf along the back. The only overhead cupboards were above the countertop, with the end piece having one screw inserted through the
fiberglass shell to fasten it. When I did my rebuild, I replaced the window frames with new ones but made the top of the frame quite a bit wider than the bottom on the rear window and two side/rear
windows. They are all at the same height, which is exactly at the bottom of the overhead cabinets, so in our trailer, there are overhead cabinets on both sides, right to the back corner, resting on the top of the window frame for support. We went with a shelf across the back, but you could do a cabinet there the same way for the full "U" shape.
When you remove the
windows, you'll find the screws are very rusted and be glad you get to replace them. The aluminum window frames are just screwed into the wood window frames on the inside, pinching the
fiberglass shell in between - that's it, but it's definitely strong enough to support your overheads.
Okay, one more tip: you don't need to overbuild the cabinets and their framing. I used 1x2's, and that's it - including the bench bottoms. Just think as you go about where the
weight is going to be, coming from which direction, and frame accordingly. Pre-drill as needed to assemble with screws. I know it sounds flimsy, but if you have a close look at the factory build, you'll find that's already stronger than the original. I covered mine with thin baltic birch plywood that you can get in 5x5 sheets locally from Windsor Plywood, and they'll stain up beautifully if you choose to go that route. Cabinet doors are the same material, just heavier ply. Bench seats etc just bog standard plywood and you're good to go.
I typed a lot, but I've just been avoiding doing real work, so hopefully there's something helpful in all of that!