The 200-4R has the advantage of being the same length as a TH350, but still getting an OD gear. It is not as popular as the 700-R4, but it used to be pretty common in car (Corvette/Camaro) swaps. Assuming a mild 350 build, it should work fine.
But yeah, if OP is planning to do a Hipo 350 build then something a bit more stout would probably be prudent.
@preSUV: Another thought for you: have you considered just getting a set of factory V8 mounts? They were available as an option in 61 and it might be cheaper/easier to get a set of them. That would give you a good starting point for clearances all the way around and be a whole lot easier than dealing with an aftermarket solution.
EDIT: Lets try that again. I told LMC 61, but it gave me 63. Here is the correct link for both I6 and V8 mounts:
https://www.lmctruck.com/1960-66-ch...e-1960-66-engine-and-transmission-mounts
