I've posted a recipe for a nice, strong 318 in the past... a little searching should find it.
The 302s are not swirl-port, they're swirl-chamber. It sounds minute, but it's a big difference, primarily since upsizing the valves destroys the swirl effect completely. Also, they're one of the worst-flowing heads out of the box for the LA engine. Max-ported with 2.02" valves (we're talking over $1000 in porting alone), they won't even match a stock 2.02" X, J, or O head. The only "advantage" they have is small chambers and therefore an easy 1-point compression gain.
The 308s are spectacular, on the other hand, being the best Mopar LA head on exhaust flow as-delivered by a fair margin. They are not of swirl-anything design. Being that your friend is doing a 318, it's pointless to open them up to 2.02" valves, as a bore notch would be required to get any improved flow over their stock 1.88" valves, and that would just lower compression. These are the heads I'd suggest, stock but milled .020" on the deck surface to get a little more squeeze. Use MP or Mr. Gasket thin head gaskets for even more compression (he'll still be well in pump-gas range; it would put him around 9:1).
The 308 heads were truck-only, used '89-'91 on 318s and '89-'92 on 360s. They're pretty easy to find in junkyards. If you stumble across some 587s or later 945s, they're not as good but they're still light-years better than any 273/318 head... just make sure to spend a little time grinding the air-injection bumps out of the exhaust ports.
The Magnum swap is a good one, too... but he'll need everything above the head gaskets, as well as hollow pushrods and lifters with oiling holes in the pushrod cups, since the Magnum heads oil through the pushrods. He'll also need a Magnum-specific intake. All in all, it's a good swap but it
is more expensive.
If he's looking for torque, use a stock 360 4v intake or one of the common aftermarket aluminum intakes. Torque requires a dual-plane; even the Edelbrock Performer I hate so much has it's place here: its ports are actually smaller than a stock intake, so it boosts low end torque. Don't go crazy on the cam; a stock 340 unit would work great, as would something with similar specs from one of the aftermarket grinders.
All in all, I'd really rather see him go 360 here but he's working with what he's got... yet there in my garage, untouched, sits a complete '87 360-4v core with which I'm doing absolutely nothing--carb to oil pan. Distance is a bitch. :doh:
If he does decide to build a new motor later, it's actually less-expensive to build a 360, realistically. Pistons for 318s are tough to find, beyond stock replacements.
If he's looking to spin the engine and make HP at the cost of low-end grunt, then we're back to single-plane intakes and a hotter camshaft, but I don't think he's gonna want that in a 5,500-pound truck.
