Thanks for the heating/cooling advice, guys, great idea! I might just let it soak in the oven at 200 for half an hour and see if that loosens things.
FYI I did put that pedal in myself, almost 20 years ago. I don't remember exactly, but I'd guess I didn't cross-thread it or anything. The mate on the other crank arm came out just fine, although it's a newer arm and possibly was replaced by a bike shop.
Re: the grinding on the crank arm. I should have said "sanding" but the point is well-taken. I'll keep my eyes on it. The inside of the arm was rubbing against the adjustable cup somewhat. I didn't have to sand much at all to get it to turn without binding when I tightened it down. FYI it's definitely not the original crank. (Although the sprocket side is, AFAIK.)
Thanks again all.