I can only say what my experience was. When I switched from a 400 to a 455, I broke the TH400 intermediate sprag(2), The outer sprag race in a TH350(2), a built Glide, an 8.2 BOP rear, and rotated the center section of an 8.5 rear on the axle tubes, sending the pinion up thru the floor.
Solved the TH400 problem with a 34 element sprag. They can now be built to be practically bulletproof.
Never tried another TH350. But nowadays, they can be built to take a lot of torque, using a heavy duty 36 element sprag/drum/race package, + other mods.
Glides can be built to withstand 2000hp or more.
For the rear end, I'd go straight to a 9" Ford or Dana 60. The base, bolt in 9" will start at somewhere around $2000, depending on what level you want it. There are all sorts of HD options available. You can save a few bucks, if you can come up with your own chunk, on the cheap.
http://www.quickperformance.com/QP-G...ge_p_4017.html
http://www.quickperformance.com/Brake-Kits_c_61.html
http://www.quickperformance.com/Comp...ons_c_558.html
http://www.ebay.com/itm/FORD-9-INCH-...hUoCeM&vxp=mtr
http://www.strangeengineering.net/pr...embly-nb.html/
A complete D60 can go for well over $3000, as can the 9".
http://www.moserengineering.com/comp...pack-dana.html
http://americanpowertrain.com/i-2091...-trans-am.html
http://www.moserengineering.com/comp...uscle-pak.html
A new heavy duty driveshaft with the big 1350 joints on each end, with matching HD slip & pinion yokes, will handle a pretty healthy Pontiac.
http://www.pstds.com/shop/drag-shaft...m-steel-shaft/
A unibody will definitely need welded in frame connectors. A full cage, with bars to the front frame rails will help hold everything together.
As mentioned, all the trashing of parts happens when you go to sticky tires. If there is a weak link, a big torque Pontiac engine will find it.