IMO
a stock 318 will not benefit much with headers. Firstly the overlap period is only 20* and the valves at those degrees are barely open. And secondly, you didn't mention 4bbl or gears, so I'm guessing this will be a low-rpm engine, and a low exhaust gas output.
So even if you got a couple, or a few, horsepower, out of the deal, it wouldn't be until up in the high 3000s nearing 4000 rpm; so say 40mph with 2.7s. Headers might help the torque output tho from stall-rpm to maybe early/mid 3000s........ but so would HO manifolds with way less hassles.
But if you are planning gears and a 4bbl, well then put me on ignore,lol. But again not much of a power-booster, and not until the engine is really cranking.
The duals tho,with free-flowing mufflers, would be a good improvement.
IMO
I think, if it was my $300, I'd sooner put it into gears or a Torque Convertor. Those will give you way more bang for the buck.
My teener came alive with a 2800. And 3.55s brought the TM up 30% over the 2.73s that were in it, and accordingly the action started 30% sooner. Actually even better than that, cuz now it actually spun the 245 tires.
Those early 9.2cr engines make pretty good torque, having more low-rpm performance than a low-compression 360 2bbl. Meaning; it doesn't take much to wake them up, and 3.55s is plenty.
If you have an early 904, the stall on those is already about 2200. So I would do gears first.
>In your case, with the 9.2/2200 teener combo; I think I would do the upgrades in this order; gears/Suregrip, 4bbl, tires, headers/dual exhaust, stall, small-cam.
>But if you have an 8.0 low-C teener with a 2000TC, then; gears, stall, tires, HO manifolds/dual exhaust, 4bbl, SureGrip, no cam unless it would be a solid flat-tappet, and with about the same net-ICA, hardly worth the cash and time for the 15hp or so that you might get.
I know you didn't ask but I know how much it hurts when I spend money and the improvement is disappointing.
Happy HotRodding
Ps; you can tell which 318 you have, by a compression test
and which year trans by the PN stamped on the passenger side pan-rail.
