As said above the frame and engine mounts are the same as a 307, I currently have the hooker engine swap headers and you basically cant run anything wider than about 5 inches, I had to go with the 3.5 inch pro star to clear the header becasue of the tube being on the outside of the frame. As far as cost a 455 is no cheaper or more expensive to build than a 350. Most things will work on both engines including cam, lifters, oil pump, oil pan, carbs, valve covers, timing chains, etc, the things that won't work are the heads (bigger cc chamber, they will bolt up and run however), push rods (taller deck=taller pushrods), intake, crank, rods and pistons. A 455 with a normal intake will also fit under a stock hood, I have a torker on mine and I made my own frame mounts and it still fits under the stock hood, barely but it does. The valve covers on a 455 will clear the a/c heater core duct work on the pass side of the engine compartment, the problem is when you get into the taller valve covers then they won't clear and as stated above you can always cut the box and flip it over and refiberglass it in to make it work or they make block off plates for it. The other thing to think about with headers on a 455 is you will also need a dual hump crossmember and a better exhasut. Good luck on the build sounds like fun! 😀