340 HP's vs headers

I would say you would be losing right around 10-15 horse with the 340 manifolds vs Doug's or TTI's.
Having said that,I had a 1970 318 dart with 340 manifolds,dual exhaust,4 bbl that ran awesome. Plus they are a cinch to install and maintain. Accurate exhaust sells head pipes to help with instalation. But as mentioned,340 manifolds are pricey. Really just depends on what direction you want to go.