I think the F70 is 235/60 14. That might be the F60 though.
The guys at Performance Wheel and Tire Inc. in Englewood CO have always been helpful and have some sales-guys around who should know the conversion. They helped me fit a set of 235/60 14's on my '68 Plymouth Fury III and knew before hand that the original wheel studs on the driver side were reverse (left-handed) thread and knew exactly what they needed before hand. They really knew their stuff. Their number is 303-934-2929.
Scott W