First, 245-50 x 15" BFGs will fit on 8" Campys in front, with a little work. More clearancing may be necessary on the left front than the right front, depending on the Pantera, and if you've added extra caster, a little more clearance may be necessary around the wiper shield and rerouting of the brake lines.
Second, Pirelli P-7R's ARE still periodically available in 275-55 x 15 and 295-50 x 15", in W speed rating (168 mph) from PPC-Carson City. Note the 'R' stands for 'Rally' and they have current date codes- they are not ancient NOS! The problem is, Pirelli makes them at the same little prototype factory in western Turkey as F-1 racing tires. Guess which gets priority? A vendor orders 10 tires and gets 3; he reorders and gets 1...sometimes, none at all. The only way this works is to call Larry and get on his list, as such tires sell within hours of their arrival and are NEVER advertised. They are also not cheap. But arguably they are about the only game in town if you want the insurance of a real DOT speed rating in a wide 15" tire.