Well Village, I don't have so many answers concerning the automatic stuff but I do have a few thoughts from my experience.
1) The Cast iron powerglides used through 62 on 6's had a flex plate but called a flywheel will fit your crank bolt pattern.
2) Starters for these plates bolted to the transmission housing not to the block like your proposed 350.
3) These starter ring gears were larger in diameter than a gear for the transmissions with the starter bolted to the block.
4) A good machinist could graft the 350 type plate in the center of the powerglide plate and the adapter plate you must make to mate the transmission pattern to the block pattern could get a window properly positioned for a powerglide type starter.
5) none of these engines were externally balanced so no need for weight addition unless dictated by a complete balancing of rotating assembly.
I keep waiting for someone to refer to a supplier of ready made componants to make this adaptation.Surely this must be a popular swap.
