Tire problem

I have a 2001 Mustang GT tire problem, I have Torq Trust II American Racing rims Front 20x8 255/35/20
Rear 20x10 285/30/20
The question is can I move up to (rear tires) 295/40/20 without any problem? Also my car is lowered about 1 1/2 inch all around. I went to my tire place to buy the tires but they said that if I order the tires and they don't fit I will be stuck with the tires approx $500.00 worth, can anyone help me with this knowledge.

Yes... you will be stuck with the tires unfortunately. That sucks because most if not all tire companies won't guarantee low profile tires. You would have to get smaller rims and tires. 18's or 17's. Or raise your Stang back up an 1 1/2" for your Dubs (20's) to fit with 35 seris tires without any problems.
Good Luck!!