Tranny confusion......Manual to Automatic

1....You need to decide if you want a 904 or 727
2....You need a transmission which bolts to a smallblock, not a slant, nor a B/ RB
3.....IF you have a console shifter, you need a trans with the bottom bolt holes for the shifter shaft support bracket
4....You need to balance the (OEM style) converter to the engine, that is a neutral balance engine needs a neutral balance converter, OR use a neutral converter and an aftermarket weighted flex plate to match the engine

..........And as above, you need to determine the balance setup on the engine. Is it an OEM style build, or has it been specially/ internally balanced, or what?