I'd use whatever I can from the Monogram kit. You just need the block, heads and valve covers from the Riv because they're so much smaller. I'd also use the Monogram intake manifold if you can file it down to fit the Riv engine.
The front cover/water pump part on the riv engine is carp. Looked nothing like the one on my real Riv, or any of the other big Buick engines I had.
They do interchange between the 430/455 and 350 on the real thing, I've done it. That's the biggest problem with the Riv engine. The intake manifold isn't that great either but it can be hidden a bit with the air cleaner and some plumping and wires.
BTW, That "star wars" air cleaner in the riv kit was only available in '67.