340 - Stock Exhaust Manifolds or Headers

Like RRR says, headers make power period. and I've heard it said that the more overlap your cam has the more significant the increase.
Same goes for the LSA. the tighter ones like a 108, make more power up top, than a wider 114 say, but this goes back to the overlap.
If it has less than 20* (off the advertised numbers),Ima thinking the HO 340 manifolds will be fine.....
If your cam has more than 40* of overlap it will respond well to headers.
If it has over 60* overlap, it would be just plain silly to not install headers.

There used to be an HR magazine test that took a stock Hi-compression 70s 340 and dyno'd it with the stock HO manifolds and compared it to long-tube headers. I forget the results but i think it was pushing 10% power increase with headers. The factory 340 bumpstick boasts 44* overlap.
I never heard anybody ever brag on their 340 being torquey with that cam, even with the advertised 10.5Scr.... so... IMO anything you can do to pick up the bottom end torque is a plus..... and so for me, headers would be mandatory