I think some guys are using 67-74 Chevelle shorty headers from Dynomax. I am lowering my truck to 4/6 and will need new headers as well. I will probably order the Hedman Elite shortys for Chevelle for a couple of reasons. 1. Flanges are 3/8 and I never had a problem with my current ones as far as leaks.
2. The 14 guage tubing keeps the noise down better than thinner wall tubes.
3. The primary tubes are 1 5/8 vs. 1 1/2 tubes on the Dynomax.
The negative is that the Dynonax ceramic headers look much better and come with a lifetime warranty. Do your homework
I hope to be converting to a big-block this summer and was told that the full length Chevelle big-block headers work with a double hump crossmember.