GT5-S

First, the DeTomaso Pantera is THE most modified exotic sportscar in the world; very very few are still as-stock. Second, since none were ever officially imported to the US (except the Amerisports), and the factory would basically build whatever a customer ordered, there could literally be any Ford engine in there. Production of the 351C ceased in '74 in the US and in '78 or so in Australia, when 351Ws were used. Very late Pantera 200s used fuel-injected 302s. So a hard answer is impossible without viewing the individual car. Finally, total production of the GT5-S was around 125 cars world-wide, all years, so your choice is limited. Estimates are maybe 50 cars in the US; if you've looked at 3 different ones, thats a high percentage of whats available. If you're in love with them, find & buy one, change the engine to your liking- one friend is pleased with his 4-cam '98 Mustang engine in his '87 GT5-S...

Jack's reply pretty much sums it up. For what it's worth, GT5-S cars were delivered by the factory with either a 300hp base motor or the 350hp "sport" motor. Exactly when they switch from the Cleveland to the Windsor is uncertain - my '85 has a Cleveland. I think the 350 hp claimed is probably a little optimistic as well...