I've run 25psi with no problems spark wise on Irridium plugs gapped at .28.
Stock coils, and both NGK and now Moroso wires.
I did add a second ground wire to the coil mount, directly to the main grounding point at the fuse box, and then another one to the battery, and engine. The coil pack wire is 10ga IIRC, and the second engine/battery ground is 4ga. (Poor man's circle earth setup, but I have to say for this old car, it makes your headlights nice and bright, and the engine seems to run better too.)
One thing about your grounds. Clean up the contacts with some 100grit sand paper, then spray them with WD40, and re-assemble. The light oil will displace any water, and help to keep them from corroding. (I've also used oxyguard, a paste you can buy for electrical connections to slow down oxidation. It's found at most electrical supply places, and you use in on home wires where the wires come into your main fuse box.) It is basicly a dilectric grease.