I have 5C heads on a 30 over 455. Its in my 4200lb GTO, they have been ported and if the timing istoo far advanced, its hot out, you have 87 octane gas, and you are near sea level (like in South Carolina) it will ping just a little bit. In Nebraska at 2400 ft above sea level it never pinged even on the worst gas I put in it. I retuned it and used a Demon 850 Carb, now it runs great on any gas I put in down here in the hot low south.
Get them ported, you will like the difference. 5C is almost identical to 6X, (2.11-1.66 valves, screw in studs etc.) stock they flow a few more CFM but its negligible. You wont have any trouble installing them on a 455. The 96's are good too, ran them a few years back on the same engine, then I put them back on the 400 I got them from and dropped it in my truck.
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