Why are your heads off to begin with?? Repair work?
The first thing that comes to my mind is a Cam.. I don't know enough about them to suggest one, but I know someone on here can.
New rockers, Roller Rockers with a more aggressive ratio could be done too, but if you put a cam in I don't think you want to change the ratio on the rockers.
You also might consider never putting the heads you have right now back on, and upgrading the heads themselves. By themselves I don't think heads give a huge gain, but when combined with a cam and headers the power increase will be huge.
With your engine taken apart it is probably a good time to plan for future "bolt on" mods by upgrading the internals.