Steve, I installed a GM crate in my 81 this past summer- fit like a silk glove. No problems with the oil pan clearance, and as you mentioned the drain plug is on the drivers side, not the rear of the pan. I'll have to look at mine again, but I dont remember the oil pan as the lowest point.Perhaps it is and I don't remember it.
Did you get a stock horsepower or a beefef-up version? Mine was stock at 250 pony's (with a 4 barrel and factory headers) or so the literature said. The main thing is I can feel the difference, and it has breathed new life into my lil jewel. Gonna go for the true dual exhaust once I finish spending $$ on the "honey-do's" around the house-or bathroom remodeling to be specific . You know, the penance for buying and restoring a car !!!! Good luck completing your new drop-in.