Without the A.C. compressor and using an aluminium intake the "big block" (actually just a taller block) will be about the same as a 307. So the springs should work. As for the headers, I know Hooker made Super Comps for the 400/425/455 Olds. The drivers side tuve came outside the frame rail. I think Rocket Racing and Dick Miller had inside the frame rails for the Olds powered G-body but grease up your rearend when you see the price. If your headers are the cheap ebay headers, you might be in for a surprise when you try to install them. Rearend sounds good, you might want to "upgrade" your driveshaft and U-jionts. I use a local truck repair shop to make me heavy duty driveshaft. Good luck and tell us more about the brand of headers and their fitment. Good luck