If I had to do it again, I'd just get the factory long branch manifolds as mentioned. They're a little expensive, but are comparable to what you'd pay for a good set of tuned ceramic coated headers. I think it was Jim Hand that did a header comparison test and found the LBM performed close to or as good as a tuned header.

I have a 69 400 with Hedman Headers [mid-grade] and they perform well but I have to be careful when traversing a speed bump. They route the left tube lower to provide more starter clearance.

Both Hooker super-comp and Doug's headers are considered top brands and as expected, the most expensive.