I put a 455 out of a 1968 toronado into a 1968 cutlass, however the exhaust manifolds to the toronado that stayed on the motor do not work wit the set up on the 1968 cutlass at all. The 350 that was in the cutlass originally has a set of exhaust manifolds that bolt right up to the 455. Is there any reason I shouldn't use the manifolds from the olds 350 on the 455?

 

They will work just fine, it's just they don't flow quite as well as the original BBO dual exhaust manifolds. If you want to keep manifolds instead of headers you can pick up some W/Z manifolds from Thornton's or another aftermarket supplier.
http://gawain.membrane.com/~thornton/65_67manifold.html
http://gawain.membrane.com/~thornton/70_72manifold.html
I have the Thornton's manifolds and they work well. I just needed to open up the outlet on the drivers side a little from casting intrusion into the outlet opening.