The only rational reason to add wider wheels & tires to a Pantera is to enhance cornering traction. It is unreasonably expensive just for posturing at doughnut shops. If you come across a pair of magnesium 10" x 15" Campagnolo wheels, be aware that the 'GTS' referred to in their description is a narrow body Euro-sold Pantera essentially identical to the cars sold by Ford in the U.S. DeTomaso was empowered by contract with Ford to sell Panteras everywhere in the World EXCEPT in North America. He chose to advertise all his narrow-body cars as 'Pantera GTS', up through about 1989. The Euro GTS could be factory-optioned up to a model sold there as the 'GTS/Gr-3'; a license-able semi-street-racer used in lower echelon International racing & hill climbing; a true Italian hot-rod, kind of like a real Shelby GT350 Mustang in the U.S..
To further confuse things, there were 138 North American cars built 1973-1974 as 'GTS' that used some of the cosmetic attachments available in Europe from DeTomaso, but none of the performance features. For instance, 10" rear wheels and ventilated brake rotors were not offered by Ford in the U.S. Those few cars are the only Panteras that display 'GT' in the alphanumeric serial number.
One of the performance parts available in Europe thru the '70s were 10" x 15" Campagnolo wheels with 285-50 x 15 Pirelli tires (widest size available at the time; 295s and 305s came later), the original 8" rear wheels being moved to the front. Along with 10" wide rear wheels were a larger dia rear 'GTS' anti-sway bar and rebuildable adjustable damping Koni shocks. Some cars' rear uprights used a double-row inner ball bearing with the 10" wheels. This is not necessary for street use but adding the rest of the parts mentioned is a very good idea! Most regular Panteras in good condition will exceed 160 mph; the Euro GTS/Gr-3 is substantially faster and has better handling. There was once a 12 pg Factory Option GTS Parts List available. Equivalents of all these old parts are available today through various Pantera vendors.
Balanced to the larger rear tires, 245-50 x 15" front tires are the widest practical tires that will fit without substantial body mods. Even then, small hammer-clearancing will be needed on most Pantera front fenders, especially if the coil-over spacers are removed (used to raise the cars for DOT headlight & front bumper height requirements- no longer enforced), and if more caster is added to front wheel alignment for high speed stability. IMHO, 245-50 fronts & 295-50 rears (along with other parts) perfectly balance the car for 'vigorous' street and light track driving. But beware- upgrading a Pantera is a $lippery $lope to step on!