The only way to be 100% certain is to try it and see what happens... I'd make sure that I could return the 13" rims if they don't fit! There is a certain amount of variation car-to-car, particularly if there has been some customization over the past 30 years... What fits on one car will not *necessarily* fit on another car.
You say you will be getting a Euro-GTS, so in theory the wheel wells are modified and the flares riveted on so a wider wheel will fit. In retrospect, I'm not surprised to hear that a 13" will fit on a non-flared car (which is what your GT5 is if the wheel well is not modified underneath your fender flare). I know of at least one person who is driving with 345/35/15's on his 10" campys on a stock body... Not the best solution, but it works for one person.
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Charlie McCall
1985 DeTomaso Pantera GT5-S #9375
"Raising Pantera Awareness across Europe"
http://briefcase.yahoo.com/gt5s_1985
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