Unported head quickness?

72 Dart Swinger,8.75 rear with 3.23 sure grip, 727 with cheetah reverse manual valvebody and stock converter, Offenhauser 360 degree intake from a junkyard, Holley 600 vacuum secondary, Dynomax long tube headers, all resting on and bolted to a COMPLETELY BONE STOCK '74 318 from a van. Picked the motor and tranny up -minus the low buck goodies- for $150 and set it in the car. The car ran 15.74 @86 mph shifting at 4500 rs (no clue on how loose the timing set is) at a very humid and hot Mopar Nats last year.
Future plans are to add a set of 714 swirl port heads ( junkyard picked for $60), freshen up the bottom end and a cam (any suggestions?)