340 exhaust manifold vs stock 318

I’ve got the same the carb and intake manifold as you along with a mild cam and 302 heads on my 318 and a 2 1/2” dual exhaust. The biggest difference I found was going from a 2bbl to a 4 bbl. Next biggest difference was going from a crush bent dual exhaust (wasn’t more than 2”) to a 2 1/2” mandrel bent exhaust. The difference with the 340 exhaust manifolds wasn’t huge compared to the other two upgrades.

340 Manifolds on 318