I plan on swappeing a 455 for my 350 in my 71 cutlass supreme conv.
Can I install headers with the 455. I probable will use 455 motor mounts and mount brackets.

 

Yes, many manufacturers make headers for that application. By the way, the port spacing is the same on big blocks and small blocks, usually they go with larger diameter on big blocks though.

 

While you can use the 455 motor and frame mounts, be aware that you don't need to. Just bolt the current 350 mounts to the 455 and it will sit in exactly the correct place. Buy 442 headers and ignore any footnotes in the listing that say they won't fit a Supreme or a convertible. While the port spacing is the same on big and small block Olds motors, the header primary pipe length will be different to accommodate the taller deck height on the 455.