The rings can certainly be stuck after that long. Whatever
oil was on them turned to gook. My thoughts are to pull the plugs and liberally spray a ton of WD40 in each hole. Let it sit for a couple hours to work on the gummed up stuff. Crank the motor with the plugs out to smear the WD around & squirt excess liquid out of the cylinders. Reinstall plugs, start it, and drive the thing with some decent load -several 1/2 throttle hits or better.
Sound silly? I did this exact thing to a frozen 260 in a 64.5 Stang & the darn thing broke free & came aroung in an hour - surprised the heck out of me...