All you have to do is look at the pictures of your frame to see that most light weight
JON boat trailers are built better than your Burro
frame. Just look at the wide end of the V where the V starts the taper to the hitch. Light weight
steel square steel tubing with a machine bend. Not strong at all and a place where you see many a Burro
and many other fiberglass trailers from all the other manufacturer's from that time period with a patch scabbed on the side of the frame to support the broken or cracked frame rail. There's your weak point!
The frame is the foundation of your trailer. You are this far into the restoration of your Burro
so why not upgrade your chassis?
I suggest you consider 1/8 inch or better yet 3/16 inch plate steel welded down the length of each frame rail hitch to tailight for added torsional rigidity and support. That's probably twice as thick as the original square tubing frame rail wall thickness originally used to construct this frame. I would look at reinforcement on the cross-members also. I would probably do the same thing to the inside of the frame rails. A little triangulation would be a good think IF the underside of the fiberglass shell will allow that with no tanks or other obstructions. After-all the Burro shell sits on top of the frame rails so this should not be a big deal. Take a flashlight and look INSIDE the square tube frame rails to see how rusty and crusty they might be on the inside as they generally rust from the inside out.
However rest assured IF this trailer chassis where designed and manufactured today there is no way the manufacturer would use this flimsy a frame design and construction with such primitive and inferior frame rail materials. As noted in one of your earlier posts you can't even buy a direct replacement axle
for this frame so that alone should tell you the technology & parts used to construct this frame is badly outdated.
As far as the worry about rust between the additional metal added to the frame and the original frame rails you can prep the metal with weld through primer and chassis paint
to stop/slow that issue before welding on the additional bracing. You also repaint the frame with a product like POR 15 to stop any future rust issues.
I would not worry about the "warping" the frame. Chances are already GREAT your frame is not square anyway and is already warped in some form or fashion after 40 years and who knows how many miles and potholes. With the above being said ANY frame upgrades you make to this frame will far outlive your lifetime so I'd say you would have "No Worries Mate". That crappy original frame somehow made it through almost 40 years!
No way "I" would stick that stock frame back under your beautifully restored Burro fiberglass shell without some serious upgrades. You frame already needs to have fab work completed to install your new modern axle to the frame so it really is time finish the job and properly upgrade your trailer's chassis!