Rene, running slightly larger OD but narrower rear tires causes no particular problem on a Pantera. All they do is slightly re-gear the final drive to the pavement and, if you have the torque available, maybe a bit more speed in all gears. I ran 275-55x 15" Pirelli P-7Rs on 8" Campys for 15 years , before a pair of 10" Campys fell into my lap. As mentioned in my 1st post, wider wheels & tires are for maximizing traction and handling. If that's not your primary goal, a narrower, taller rear tire will work fine, look OK and can even be 'adjusted' outboard a bit for better appearance with wheel shims.There's plenty of clearance for even giant rear tires with proper wheel backspace. Front tires are a completely different story.
The 285x40-15" tire was the first really wide street-type radial tire available to the public, so thats what wound up on Gr-3s with the optional 10" rear Campys. If 295-50s or 305-50s had been available, DeTomaso and the racers would have gone with those. Many Gr-3s ran gumball race tires on track or during hill climbs, or in some cases they used treaded racing rain tires because with most Gr-3 club-racers, they WERE interested in maxing traction, not necessarily in European street legality or tire longevity.