Sounds good. You seriously gonna roll that bad boy with 4 wheel drum brakes and a single pot master? Brave, very brave.
As far as the 200R4 is concerned, properly built, it will stand up to your engine. I've put them behind some really stout small block and big block Chevys including a few 572s. Notice I said "properly built". The ones I installed were done by California Performance Transmissions (that would be Art Carr himself). The key to the hydraulic GM auto-overdrives is the the TV cable adjustment. It's critical that it is adjusted properly and the carb bellcrank pulls the cable correctly.
In your application, gas mileage isn't the goal, but driveability will go way up.
There is the option of going with the 4L70/75/80 transmission, but as stated, you'll need to run a wire harness and computer with it and the computer will require "tuning". There some good systems out there, and there are some not so good ones, too. Generally, by the time you get one of the computer transmissions gone through, get the right parts, converter and electronics set up and dialed in, you could do a 5 speed for about the same price.
Don't even worry about the six speeds. Nobody needs a double overdrive transmission if everything else is planned out right.
Oh, as far as that rear suspension is concerned, check out Global West https://www.globalwest.net/
I knew these guys back in the day and they have a well designed product and really know their stuff.
Hope that helps.