383. I have the old exhaust but it looks like its 2" - 2.25". If I get stuck I might use them and add adaptors to the new 2.5" system for the rest off the year.
You may be able to cut the headpipes you have now, to where headers would terminate.
Take those pieces to a muffler shop and see if they can bend two pieces that copy the ones you have.
When I put my headers on, I made cardboard templates for two pieces of pipe and took those down and had two pieces bent. Then I put my exhaust on, that was before I bought the JEGS kit.