As I noted in the '02 response you've referenced, it was in conjunction with noted-Pontiac researcher Pete McCarthy that we deduced the changeover date from the 5C-series of cylinder head castings to the 6X. I've since been able to further prove that point by documenting a handful of '75-model year Pontiacs that were factory-assembled with a 5C on one bank and a 6X on the other. So far, each example had a 5C produced in late January (signified by an "A" as the first letter of the cast-date) and a 6X produced in early February. And it seemingly coincides with Pontiacs built in late-February 1975 and through at least some of March.
If your Trans Am's production date and its cylinder-head cast dates are within a reasonable range of the stated timeframe, it appears your particular Pontiac is another that was originally equipped with mismatched cylinder heads. That doesn't present a performance issue, however, since the castings share the same general port and combustion chamber characteristics, and assuming that the secondary-application stamps of your castings match, you should find no functional difference between them.
While the mismatched cylinder head anomaly might make your Pontiac rather unique, I don't expect it to add to the perceived value any. However, I urge you to move forward with the resto and I suggest taking countless pictures throughout the entire process. It will help document your particular Trans Am's original attributes, it will also answer your own questions when it's time to refinish or reinstall certain components.
You are welcome to share your comments and experiences, and keep readers abreast of its general progress by using the forums found on our website at www.highperformancepontiac.com.
Good luck with your project and when it's finished, send us a few photos.