I've been preaching the low gain with headers on stock type combos for years, but the 318 guys with their 8.0 smoggers all preach me wrong.
I'll say it again;
for headers to work right, it takes the entire combo; most especially the overlap period and the Effective compression ratio.
They are never a "bad" idea; but on a stocker, rarely worth the trouble, compared to a hi-flo Dual Exhaust, which will make all the difference.
Headers may make a difference on the Dyno, I can't say , but 10hp at 4400 is as good as un-noticeable from the driver's seat... when it climbs at a rate of 2 hp per 1000 rpm, peaking at about 40/50 mph with hiway gears..........
The factory 318 LA cam has 20* of overlap on the advertised, which at .050tappet rise is already looong gone. So headers will never get you significant power at rpm, over and above the meager results they otherwise make..... on the stocker.
The Mopar 292/292/108, cam on the otherhand has 76* at advertised, which works out to 32* at .050 so this cam will respond very nicely to headers. Even the mighty 268/276/114, 340 cam has 44* advertised, and by .050 it is down to just about 2*. Yet when you put headers on a factory stock 340, the power increase is likely to be in excess of 20 hp, over and above what the HO manifolds do. So some of that is in the individual pipes, and some is in the overlap. Just keep in mind that the power peak is up near 5000, so forget running hi-way gears.
anyway; rant over.