My boat was originally purchased with a 455 in 1975. It was replaced in 1978 by my father in-law with a 454 oval port 325 HP 8.25:1 engine (the Olds seized).
The engine mounts for the 455 (in my boat) used two bolts horizontally on each side. The Chevrolet engine uses two sets of bolts each side vertically.
The engine cover did not have to be altered (as far as I know) to accommodate the BBC.
Jim Brock has some Chevy 4 point engine mounts for sale on his web page.
http://members.aol.com/doug1yates/cyclone/webdoc3.htm