"If you were buying comics in 1975-76 you would know this and quickly realize that the New X-Men is a Modern phenomonen that had little or nothing to do with the Bronze Age."
Sorry, CI, but on this one you're wrong. At least in the Chicago area, the "New X-Men" were the absolute hottest thing out there from at least X-Men 96 on. As an example - I had a booth at the Chicago Comicon in 1977 (I was 12, that's a whole other story) and my friends and I sold all of our X-Men for 3x and 4x Overstreet Guide (which, admittedly, at the time was about $2 each).
I can tell you stories about buying and selling X-Men 94s at Chicago shows for good money before Byrne came on board. Here's the bad one...I passed on the original art for the cover of X-Men 94 in 1980 for $200. Too much money for my 15-year-old wallet... .
But I agree that GS X-Men 1 is too late for the Bronze Age. I'd go with Conan 1. I'd put the end later than most, Crisis 1 and Secret Wars 8.