Largest rear tire on a 77-79 Cougar (TBird)

I am trying to find out the largest tire that will fit on my 79 Cougar. I presently have 275/60/15 on read, but have not had it on the road. The front tires are 225/60/15. The wheels have 4 3/4" off set. Does anyone know the largest offset that can be used? It seems that the limiting factor in the rear is the frame rail in front of the rear axle. Is that right?

Have you checked out the tech pages on the tires for a Torino? Should fit about the same as the Cougar, Grab a tape measure for sure.