It seems to me that the Euro GTS have a more powerful engine, 330 or 350 hp vs 270 for the US GTS. Am I wrong?
The GTS engine differed from the Pantera & Pantera L via the installation of the GTS exhaust system and a 650 cfm Holley double pumper carburetor. That's all. To my knowledge De Tomaso NEVER swapped pistons or cylinder heads. So the engines had whatever compression was standard for the particular long block being used. 350 DIN horsepower (346 SAE Net horsepower) may have been realistic when the first GTS models were introduced in 1972. They would have used a 1972 Q code long block (8.6:1 compression) , Cobra Jet cam, high port (i.e. 4V) heads, and added the 650 Holley and GTS exhaust. However the compression ratio dropped in 1973-1974. Power would have diminished ... but De Tomaso continued to quote the original 350 horsepower measurement, just as they continued to quote 330 horsepower for the Pantera and Pantera L
When US manufactured 351 4V engines were no longer available De Tomaso substituted the Australian 351 4V. The Australian 351 4V had 8.8:1 compression, was equipped with the same camshaft as the 351 2V, and had 2V cylinder heads (lower ports and smaller ports tuned for lower rpm). Ford of Australia falsely rated the engines of ALL 351GT Falcons at 300 horsepower whether it had been imported from the US or manufactured domestically. Over the years I've seen other ratings from 215 DIN horsepower to 260 DIN horsepower for the Australian 351 4V engine. For what its worth, 300 DIN horsepower peaking at 5400 rpm is equivalent to 244 DIN horsepower peaking at 4400 rpm (the 2V heads are tuned 1000 rpm lower than the 4V heads).
At best, the "true" output of the Pantera GTS equipped with an Australian engine would have dropped from 350 DIN horsepower @ 5800 rpm to about 290 DIN horsepower @ 4800 rpm ... by virtue of the fact that the 2V heads are tuned about 1000 rpm lower than the 4V heads. Some owners claim De Tomaso did not swap camshafts, if this is true the output would have been even lower because the valve lift would be about 0.400 inch instead of 0.480 inch.