Well, as the guy who used to install cabinets and deal with all that, I would scribe the edge to fit the crooked wall so the homeowner didn't blame me.
I don't know if he's still around but there was a guy in the Orem/Provo area. I think his name is Russ and he went by Dr. Jeep. He parts out TJs and would probably have one. I don't think I've ever seen one in a wrecking yard.
Are you looking for a trail rig that's streetable or more or a dedicated rockcrawler?
This one is more "built" and fewer miles/less money.
https://classifieds.ksl.com/listing/67232612
My thought is that just upgrading springs and axles doesn't necessarily increase the capacity of the trailer. I would think they are designed accordingly. But I really don't know. Sounds like you have time and could look into that with the manufacturer.
I believe the 411 swap is for the '96-'99 Vortec. I don't think it applies to the TBI style and as has been said, mods are difficult for the TBI because of the computer.