Easiest/Cheapest way to a 4bbl 318?

FWIW I bolted a set of 302s on my stock 318 bottom end with a Lunati Voodoo 60401 cam and a 625 cfm AFB on a Performer intake and it was fun even with 2.76 gears... However my engine wasn't in good shape and it was really soggy on the low end from low compression (at high altitude as well) and worn rings. You seem to have gotten one of the "good" ones though, taken care of and built on the better side of tolerances.

Since you are using 302 heads I'd say go with a Performer (or similar) intake, the bigger ones won't match up as well and the head ports are too small to take advantage of the flow. BUT there is extra power to be had from enlarging the intakes to 1.88s and working the ports to suit, and in that case an RPM-level intake manifold would have greater benefit. If you are able to change gears to even just 3.23s a cam like the Voodoo 60401 I used would be perfect, or the next level up also if you are getting bigger valves and porting. Finally headers would be nice but IMO they aren't really worth it because the only ones that fit well are pricey and the increase in power at this 'level of tune' is on the small side. Find some junkyard Magnum manifolds from a Ram/Dakota/Durango etc. and massage them to fit or even just some OEM 360 car manifolds would do the job...

EDIT: Just a "HEADS"-up haha bolting a set of closed-chamber heads on a short block with a big quench distance might increase your chances of detonation even if the compression isn't that high in the end.