If you hae the Quadratrac now, you'll need to swap output shafts to go to a Dana 20. The ouput shaft for the Quadratrac is unique.
A used engine will be your cheapest repair, if you can find a good one.
If you want to rebuild and your 360 block is damaged, it will almost certainly be cheaper to find another core engine to start from. If just the cam bearing is damaged, clean the block and reuse it. Your machine shop should advise you about this at no cost. Take the block to them and ask whether it's ok, and if it will need expensive extra machining.
Likely you can take the frame brackets from the 350 Wagoneer and they will bolt on. The 350 Buick is a great engine, lots of torque - meant to move heavy Buick passenger cars. The main downside to the 350, as I see it, is that you'll have the wrong engine in the chassis, and the Buick setup has some quirks. Parts are less plentiful for the Buick-engined FSJs than the AMC-powered trucks; things like a radiator will be harder to source. Kaiser used an unusual adapter between the engine and the TH400 that was unique to Jeep, and it has its own torque convertor. If you need a torque convertor, you'll likely have to have your core rebuilt. There may be no source for new, compatible flex plates. Likely other issues. Things will be simpler if you stay with AMC.
Or, if you want to go all-out, find a Chevy 4x4 donor and replace the engine, transmission and transfer case together. The Chevy swap is pretty popular, and would likely sell easily if it were done well. You could go with a 700R4, which is more efficient than the TH400, and gives you an overdrive gear. And parts would be easy to find and cheap, as long as you keep track of what was used where.
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D