Me thinks those stock 318 exhaust manifolds will restrict the exhaust flow on those 340 heads, unless you enlarge them. All the stock 318 exhaust manifolds I've seen have the center ports siamezed and all the 340 and 360 manifolds don't
The first 340 motor I built for a customer years ago was going into a 1965 or 1966 Formula S Baracuda that had the hi Po 273 motor in it originally, I told the customer it would need a good set of headers and he ended up installing the motor with the stock 273 exhaust manifolds to get it running, needless to say it didn't run that good with that stock exhaust system on it
He sold it and the next guy fix it with headers and dual exhaust