I can't find the old post on this but...
If you have a 4 bolt 13 inch wheel you can start with getting a 5 bolt hub for 14 inch rim.
With the wheel off and the trailer frame supported you can position the axle
arm so that the square center hole is aligned with points horizontal like this <> That is how it was aligned when new. This allows you to establish height of "original" axle spindle on the end of the arm.
Positioned like this with the center shaft <> horizontal it also allows determining the original "down" or "up" angle of the axle. Which will provide a starting point for that angle on the new axle.
The older axles were trailing arm, that is the arm "trailed" behind the axle cross member. Later axles were "leading" arm as the arm was pointed forward of the axle cross member. Wheel trails or leads the axle.
The bracket has to face the right direction so the arms are pointed forward or rearward. The bracket is longer in one direction and typically on a scamp
will be snugged up into the place the frame goes up for the raised rear floor.
You may well on a 1979 have a leading arm axle which they don't really make any more so they "turn the axle around" and make it work. This can involve welding a piece into that corner in the frame where the axle was and attaching the axle to it. That can have the effect of positioning the axle "lower" which will raise the body but won't raise that axle cross piece which is your true clearance. It may move your wheel lower in the wheel well allowing for a larger wheel.
Note: it may be that the trailing arm is the one older scamp
will have that they don't make anymore but one of those two, leading or trailing that older scamps used is not commonly made now.
Your limit on increasing the actual ground clearance as in the distance between the lowest point on the frame (the axle) and the ground is the size of your wheels and tires
. That size is limited by what will fit in your wheel well and not scrape when it moves up and down on bumps.
Getting your body up so it is less likely to scrape the back or front going over dips and inclines can be done by having a spacer between the frame and axle. Which you may well need anyway to transition from leading arm to trailing arm.
I would recommend getting the frame on jacks, with wheel off and find the original axle arm angle and spindle height. This gives you a starting point of what height was when new. I'm pretty sure an original axle is worn enough to have dropped the height a lot since then.
Changing wheel size means buying new wheels and tires
but nothing uses the old 13 inch 4x4 bolt pattern anymore but small boat or snowmobile trailers.
If you change the bolt pattern of the wheel you will need to have an adapter plate made to put the different bolt pattern of the wheel onto the back as a spare. Need a plate with bolts coming out that line up with the new 5 bolt pattern that you screw to the back through holes using the built in 4 bolt pattern. I also strongly suggest have a support piece for the plate welded to the bumper since the wheel will be a little further out from the wall.
Last but not least order that sucker with brakes! The axle has to have a mounting plate for brakes
built into it. Even if you decide to save the few bucks not buying the brakes
themselves right now you want that plate to make it possible to add brakes later. I tow with an older boxy Ford Escape
V6 and it stops faster towing the trailer than without due to the trailer brakes. Trailer brakes are really really REALLY nice in mountains or hills or when a traffic light
turns yellow. Or wet pavement.
Also 13 inch wheel hubs with brakes are not common, you have a bearing go and damage the hub and the 14 inch or 15 inch hubs are common enough to be in stock, good luck finding a 13 inch hub equipped for brakes in stock. Two week wait would be more typical.
I ordered my axle for 13 inch wheels with brakes, really regret not going for 14 inch, the cost of new wheels and tires
would have been more than offset by larger selection of tires and rims, plus I wouldn't have to haul around a spare hub to avoid being stranded waiting for a new 13 inch hub to be shipped in. Just look at etrailer for tires and rims to get an idea of how much larger the 14 and 15 inch selection is. Lot of 14 and 15 inch items, 13 inch 4 bolt? Not so much.
I can't imagine ordering a new axle without brakes.