The 403 is and was intended for bigger cars, delta's, cruiser's, and so on. It basically was a station wagon engine, low hp rating big low end torque. Olds 260-403 engines are physically bigger than the Chevy sb's so the assumption is that they are a "big block" but really not. The 400, 425, and 455 are big blocks because they have a taller deck height {10.625 vs 9.33} to accommodate the larger stroke on the crank. Regards to the 455 swap..it is as simple as swapping motor mounts 350's onto the 455 BUT, remember the 455 will sit in the engine bay about 1" taller, doesn't sound like a lot but think about how much closer the air cleaner will be to the hood. One way to correct this is to get the engine mounts and frame mounts from the SAME 455 that it was from and swap BOTH mounts. I say go for the 455, minor improvements to the engine like intake manifold, air cleaner, headers with true duels (scrap the crappy manifolds) cross over pipe is a plus, recurve the distributor, put a GOOD carb on it, plugs and wires and you will not be disappointed!! You would need to do a lot more to that 350 to make it move like that 455 would, they just don't make the torque needed to make that big car go. Oh, one more thing and prob a very important one, replace the oil pump. It will insure that the engine will have good oil pressure plus its a cheap insurance policy for problems that could be avoided later on.
