I have put a 383 in my 56 Coronet, but then I went with a mustang II front end.

Definitely used the 70's truck oil pan. Mine was from a 4X4 for a rear sump. I built my own mounts and actually just used the rubber cushions from a late 60's big block and built up the lower mounts on the cross frame (fat man fab frame).

I have a 727 in mine and was acutally able to use the stock crossmember for the 56 Coronet. I had to weld on a mounting bracket from another b body (just the part that holds the transmission mount) onto the existing crossmember (with a few minor cuts).

With powerbrakes I had to move the master cylinder up 5", but probably wouldn't have had to do it if I had left the old brake system in it.