you didn't say, but going by your avatar you have an early A ('66 or earlier) and you say you are using the stock 273 manifolds? Anything you do for an exhaust system will be a negligible improvement for a 340 trying to breathe through those exhaust manifolds, sorry. First order of business is to eliminate that bottleneck. Lots of threads on here about manifold/header options for early As, the biggest hangup is whether you have power steering or not. If you don't, your most cost-effective solution is a pair of Magnum manifolds off a Ram or Dakota (possibly others) that generally fit manual steering with minimal tweeking.
Then you need to mod your transmission crossmember for dual exhaust. Lots of threads on here about that, too.
Then it's a matter of getting some head pipes bent up locally and add the exhaust system of your choice.
Many other options too, but those 273 manifolds really should go...