If it was fouling plugs, my bet is on oil fouled (worn out rings).
Take the engine to a reputable engine building shop. Tell the shop it's fouling plugs and go from there.
Shop will likely quote you for a basic rebuild (they need to tear it apart to really get an accurate diagnosis).
A basic rebuild consists of new piston rings, rod and crank bearings. I'm sure the pistons and crank themselves are fine.
Cams are cheap so you might want to replace that while you're in there ($100+).
The heads will be looked at as well as the valve seats will likely need to be re-cut.
The same shop probably rebuilds carbs (or knows someone who does) so get a quote on that as it will need to be gone through after sitting for 40years. If it's a tri power, three carbs WILL get pricey to rebuild.
Been a while since I've had an engine freshened up but you are looking at around $2k+ (all depends on what is needed).
You are ONE lucky SOB to get handed a 66GTO. Wish I had your problems right about now.