A good quality rebuild will net the most power. I would balance the rotating assembly (crank, rods, pistons with balancer and flywheel) and use a balance shaft eliminator kit. Your biggest power gain can come from the cylinder head. A good 3 (or 5) angle valve job with some mild bowl blending (nothing too crazy - a full port and polish is NOT necessary, just smooth out any casting flash/flaws on the intake and exhaust ports, give them an even looking finish). You could blueprint the engine (check/adjust deck height, CC the combustion chambers, make all cyl. volumes as even as possible) but it's a lot of work for only a 1-2% gain. ARP head studs and a good head gasket would be a worthwhile investment and refurb/replace the cooling system.