First thing you need to do is get the seller to confirm that front wheel width - if it really is 10", they're not Ford-original GT350 front wheels (the GT350 came with aluminum 19x10.5/19x11 and the GT350R got carbon fiber 19x11/19x11.5).
The 19x10.5" fronts are supposed to be +30 offset. If that is correct - and I'm pretty sure it's good information (
"Official" GT350 Wheel Specs), they should "fit" but the wheel and tire will stick out a little further past the sheetmetal than mine do. Can you live with a little "poke"?
I have some doubts, based on your 1.5" of lowering and what I can show you about my own car, which is not lowered in the following pictures. It's on 18" wheels, which give another half inch tire to sheetmetal clearance in any given tire size than 19's will.
In this picture, camber is -1.9°, so if your camber is closer to -1° the upper part of your tire would be another 10mm or so further out. You will have more than half an inch more wheel to strut clearance than I've got (which is hardly any at all), but that's not going to help your outside situation.
Tires in the above picture are 285/35-18 but on their 18x11 wheels they actually measure close to 300. If you run 295's or 305's, the tire bulge may be a little wider, but only a little. Tire tread width is probably going to be the limiting factor in what you can run. I suppose if you're braver than I am about DIY bodywork it'd be possible to very carefully roll the fender flares out a little.
What's a "gt250"?
Norm