slant 6 to 318 conversion

Ok, so it doesn't matter if its a 904 or 727, but I will need a small block tranny to go with the engine, correct? Also, aside from the standard motor mount changes, is there any differences mounting the small block tranny in place?

Incorrect and correct. It does matter if it's 904 vs 727 slipyoke size and driveshaft
length wise, but yes it must be SB. As mentioned, if you're changing rears to 8.25/8.75
now is the time if you need to change driveshaft length, 8.25 needs approx. 1.6 shorter,
8.75 needs over 2" shorter. If you go 727 you will subtract another 4".