jb--I swapped out my 327 for a 250 in my '68 1/2 ton about two months ago. I know you're going the opposite way, but should be just the reverse of what I did. I swapped out motor mount towers and that was it. I had to move the towers up one bolt hole(I think). A small block should fit with no problem. I don't know if I really had to swap out the towers, but the previous owner welded one of them to the motor mount, so I did anyway. I don't know about the tranny. Sorry. Hope this helps. John.

1968 Chevy C-10, 250 and 3 speed on floor, Hedman headers, Offy intake, Edelbrock 600cfm 4 barrel, HEI
1966 Chevy C-10, motor and tranny went into the '68. Waiting to be restored.