thats what I did to my 72 skylark. the A-body frame is all the same on the chevelle, skylark,pontiac GTO, and olds cutlas.
was your car a 455. if wasn't use a chevelle trans cross member, the buick 350 trans cross member is weak and the ends that bolt to the frame are mounted in rubber, which flex alot under load. if you use the th350 or th400 the drive shaft will work in them.
the engine cross member has all the holes drilled for all the different engines. so what i did here ,i used a bare sbc to line up the right engine mount holes, thats if you use the old style engine mounts ie the chev brake away mounts, or do the same with the newer style mounts but they will over hange on the front of the cross member, so you'll have to add some support to them.you'll also have to change the fuel line to the other side of the frame rail. the fuel pump is on the opposite side. I used the same starter wire harness . buick runs there on top of the intake .i did the same with the sbc , because i was runnig headers, the chevelle runs theres on back of the engine and when you run headers the wires almost touch and some times burn!!! and you have to re wire your alternater and ign wires and replace your fan shroud, and exhaust. that should almost cover everthing.