Will the existing manifolds work when switching fro the rocket to a 455 or will they have to be changed?

 

intake manifold will not swap over.
exhaust manifolds will.

 

Yes, they will fit, if you have single exhaust you will have to rework the crossover.
The main thing is they are much smaller and restrictive. If you have dual exhaust you will have some align problems.
Gene

 

I replaced the 350 Rocket in my 70 convertible with a 455, 68 police intercepter kit, headers. Dropped right in.

 

Don't use the 350 exhaust manifolds on the 455 way to small and restrictive. Use headers of W & Z headers. You can find them on e-bay

 

While they will physically bolt up, in addition to being overly restrictive, the other reason not to use SBO manifolds on a BBO is that the exhaust flanges are in different locations. The BBO manifolds (at least on the A-body installations) are designed so that the exhaust flange is at the same location as it is on the SBO. Using the shorter SBO manifolds on a BBO will put the flanges higher and further outboard, which makes custom pipes mandatory.