I think the chevy crossmembers are the same for 55-59. Don't know for sure... never had a chevy. I thought the chevy used the donut mount and had two sets of holes, the closer set for the 6 cylinder, and the wider set for the v-8. Don't know for sure.
Only know about the gmc crossmembers because somebody had welded a chevy one in to my gmc chasis and put a chevy 235 in there. I had to locate a 58-9 gmc crossmember, and I had to cut the chevy crossmember in pieces to get it out. All that for a truck I ended up getting rid of before I put the 302 gmc in it!
I really don't think you need to swap crossmembers. I think you could probably set the engine in there and drill the holes where you need them in the gmc crossmember. my .02
Getting the crossmember out is the easy part. Trying to get it back in is the hard part.
If you do go for the swap, Get a lot of patience, a big hammer, and a couple heavy duty ratchet tie downs. The frame has a tendency to open up just enough so the rivets or bolts won't line back up. Good Luck!
John
