[CobraII76]

Start with the head question most aftermarket heads and all 351W heads have 1/2 inch holes for the head bolts all 289-302's have 7/16 head bolt holes. ARP makes a bolt set that uses 7/16 bolts with built in 1/2 washer to put aftermarket and 351 heads on the 289 302. Head gaskets intakes headers etc stay the same. So shop for your future engine build in the heads and use them on the 289 with the ARP bolts.
Next question got any idea where the timing on this thing is set? a lot of times when people install a performance cam they go full advance on the crank sprocket thinking it will get them more power and they tend to lose good low end and seat of the pants feel for higher revs that never make any power. I'd geta light on it and see where it's at and tune from there. Get me a carb list number off that holley and I'll check the jet size for you. you may want to go up one or two sizes could make alot of change in where the power comes in at. You could also try the old secondary trick of getting a small nut and bolt into the slot and making them open with the primarys to check the secondary function. I would start with timing and go from there.