yeah I wouldnt worry about clearance issue's. You mite wanna see if the headers will bolt up to the heads! Like Realmongo said 350's are not interchangable with 455's!
The headers are designed to position the outlets at the same position in relation to the frame. The 455 headers are longer than the 350 headers because the block is taller. By bolting the 350 headers on the 455 block, they come up and out and their outlets wind up in the wrong location.