joshhol: I am going to say YES on that one: my parents changed the oil religiously on our 4.5 and it only had ~40K miles on it when they got it in the late 80's, and the rad developed a leak - in its years before we parked it (before I did all the work on it and got it running) it never overheated, but did get hot a few times. My cams and rocker arms were a MESS. This is the only thing I can attribute it to. Therefore, I am going to guess that these engines suffer severe damage to the top end when they get too hot!
If your 4.5 isn't rusted at all, I'm sure plenty of people here *cough*me*cough* would gladly buy it off you for $500 - even if it smokes as badly as my dad's GMC p/u with 180K miles!