So on a stock 4 speed car, the simplest way to accomplish a swap to an OD trans would be to modify the OD tranny's bearing retainer, and use a stock OD shifter?
Yes, all you need to modify is the input bearing retainer and it's super easy if you have access to a lathe. There is no interference with the mounting bolts. There is no gasket, so just clean it up and smear a bit of gasket goo and you're done!
The shifter is even the same, only difference is the 3/4 tab points down instead of up and you might need to modify the shifter rod. Easy stuff.
Yes! I have an OD tranny with a 3.23 LSD in a '70 Challenger with 26" tall tires and love it!
The OD trannys have a 3.09 first gear which helps get you out of the hole a little faster and the 0.73 drops the final cruise to down to a happy cruising level.
Some people complain about the gear ratio spread but I've never minded it. FWIW the gear ratio spread is almost exactly the same as the GM 700r4 automatic.