What is the engine in and what kind of emission equipment does it have? This stuff makes a huge difference. Late '70 truck 318's are not smogged down and have decent heads, so they can benefit from adding a 4 bbl and a mild cam, while the emissions crap and heads on an 82 Fifth avenue really can't make use of the extra flow.
Most 318's run flattops with 8.4 or 8.5 CR's, but the heads vary wildly. Milling them 0.030" or so (any more will require adjustable pushrods and milling the intake), porting and port matching, a small CFM (500-600) 4 barrel and a 360 truck cam will give a decent increase. This is exactly what I did with my '78 aspen (thought he heads are 593's off a '76 truck engine). Cost me about $700 total ($260 for the Edelbrock 600, $120 for the Napa cam & lifter package, $100 for the head gasket set/timing cover gasket set/timing chain, $100 to have the heads milled, and about another $100 in misc small parts. I did the porting and port matching, and I already had a police interceptor 4 bbl cast iron intake (also got ported).