I used a Buick flex plate from a 62 Skylark I believe, bought it from one of the members here. Works perfect. Had to be drilled and spacers used to adapt to the Th200c TC which I believe would be the same as the 4R TC=std GM. You can also use the original roto flywheel but it also needs to be drilled for the TC and has un-needed mass since you now have a TC. As Joe says the adapt plate and a centering pilot-D&D can help out a lot. A little pushing out of the tunnel is all that’s required with the 200c while to repeat, tunnel mods are needed for the 2004R. Get the B&M lock up controller, you want to lock up, Easy/simple install for the controller.
Some members have done the 200 4R swap I believe and there is info here on it. Do a search on Google with oldsjetfire in the search subject and you should be able to find something. You can’t do a search on this site.