Again I'm not really doubting you, Rifraf, but I thought all the late model 350s (5.7s) were 4-bolt mains and used the same blocks. I do know that one of the issues with vortec heads from junkyards is that those from heavy trucks were worse flowing and did not have the potential of the light truck heads...seems there was a thread a year or so back where this discussion came up and someone posted casting numbers so you wouldn't make a mistake. You are absolutely right about valves designed for much higher exhaust temps.
By the way...4.3s use the same motor mounts as small blocks...but not the same as the 2.8s. It is not difficult to change the pads so they fit.