The tranny is cantilevered. Not supported by a cross member. I believe the bell housing is supported by the cross member. (truck bell housing). No changes there that I know of. Novak is very informed about SM420 and all other transmissions. SM420 used on jeeps and rock crawlers galore.
You will see on the vendor site that Tim linked, a flat floor cover and a one with a hump/raised area. Those are the one's we are speaking of.
As was mentioned a year ago by Cletis and others, SM420 has compound low. Good for certain uses. You will most likely start out in 2nd most of the time. Also several people here would like to have that OD 3 speed you have/had.
Sounds like you have your ducks in a row.
If the bell housing mounting face for the transmission is in the stock location after PO installed a 57 motor, then a SM420 shifter will be in the center of the hole in the cover on Tim's link.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn