If you are interested in a longer tube header, check out FPA headers
. He offers a step down header that can fit almost any combination of parts that you have and one complaint that I read is that they hug the body "too tightly"
. Not a bad problem to have with a lowered car.
I have set, just not installed yet. Quality looks great and he has a good reputation.
Good luck with the build.
1968 Fastback: 331 stroker, FTI cam, N351 Heads with a few upgrades, 4r70w w/3600 stall, 9" locker w/3.70 gears