Standard torque nowadays for a 1/2" fine thread lug nut is 100 ft. lb. I think Ford's steel alloy back then was as good as today's, and 50 ft. lb., to me, may let the wheel 'fret' a little and work its way loose. Borrow a torque wrench if you don't have one and torque a few lug nuts to 100 ft. lb., then loosen with the crank handle or whatever you normally use. See how it goes.
P.S. Harbor Freight has torque wrenches for $9.99 with a coupon from a magazine ad. I've tested them with an electronic torque gauge and they are within a pound or so at 100 ft. lb.