I am not 100 percent sure but I believe there is a difference at the cranks rear flange.
You are correct sir. The mating of the engine and trans prior to 1962 is completely different than 62 and later. Also different is the trans, cables/ control, driveshaft flange emergency brake etc. They did almost a 100% redesign of all mechanicals in 62.
On both, been there done that more than once, use the later 727 and buy and use a decent floor shifter
If you do this complete swap you will need to change the driveshaft also, Mopar use the old ball and trunion type front U joints in the automatic cars until 1965, they switch them on the stick shift cars in 1966.
