if you tried all matching parts, and it still doesn't fit, i'd suspect that maybe the frame is tweaked or something. all Chev V8's from the '56 265 to the new 8100 in the new all trucks have the same 3 bolt mounting pattern on the side of the block- even the 348/409 "W" series used the same mounts- so that would not be the problem.
get your frame checked out and go from there.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?