A lot of guys are recommending the Toyo Proxes S/T rear and Proxes 4 for front. The GT40 replica guys with 15" wheels are also recommending Toyo's as the best available 15" performance tire out there.
A search here on the PIBB will get you a number of previous threads on the subject.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.