Headers VS Manifolds
If you're not chasing ET stick with the manifolds, one of my friends with a 64 Valiant was running 12.90's with 273 logs and iron 4 barrel intake on a 360, it was a standard rebuild with KB107's, cut the heads for 2.02's and has a 284/.528 solid, he had an old factory AFB on it too.
The ET bug got him so he swapped to TTI's, an Air gap with a demon and runs 12.2-3's, he just put edelbrock heads on but hasn't been back out yet- maybe 11.? now, it may be faster but I thought it was cooler when it ran high 12's and looked like a stock 273 under the hood.