What exactly are we looking for here? Is it the boxer who had the most fights, when he was a HW? Or the the guy who had the most fights, against HW opponents? Or the one who took part in the most fights, where both were over the HW limit?
Could Young Stribling be a contender for this honor? He had 250+ fights and while he has some recorded weights of 175 or below a good majority of his fights seem to be at heavyweight (it’s unlikely he dropped down to light heavy in bouts with no recorded weights when they took place between fights where he’s clearly a heavy). But I’d have to count them up. Earlier in his career he did fight in the high 160s and low 170s, occasionally bouncing up to above 175. EDIT: By my count on boxrec, I found 129 fights we can reasonable assume Stribling came in over 175 (a few of which however his opponent was at or below the light heavyweight limit so it’s not established how we want to count those). However, as noted, several of those fights have no recorded weights — if his last recorded weights before and after any bouts were at heavyweight, I counted those as heavyweight fights although he wasn’t a huge man and it’s not impossible he dropped from 180 to 175 and came back up so those aren’t 100% guaranteed.