Standard fitment for the 5000GT was a 650-16 crossply, but today, a radial is probably a better choice for this rare and powerful GT, a 185VR16 would be a suitable size.
The 5000GT was a limited-production version of the 3500GT, using a detuned racing V8 from the 450S sportscar. Usually built to order for rich customers, using bespoke bodies by various design houses. Rarely seen today.