Richard, the main reason that late model (after 1975

) cars have HEI and large plug gaps is for emissions.
I've found that older cars do best running the gap the engine was designed for. Compression ratios and major mods aside. And yes, even with platinum plugs. (Actually, I like the standard plugs much more than platinums in older-design engines, like ours.)
I'd run closer to .035-040 in your engine, or even tighter to what Brett recommended. Pertronix and that coil you have aren't a full-boat HEI system. Better than stock, but the gap doesn't need to be increased much, if at all.
Others may have different opinions, but .040 is what ran best in my KR with its aluminum 482-inch side-oiler, 10.75:1 compression.