for a '65 coupe, what is the best backspacing for (front) 17x7? i want to have them nicely tucked in!

 

4.25 but you might get away with 4.5. It's the fatness of the tire that generally causes them to rub with these backspacing figures. So, shop carefully for the right tires to fit your wheels to get the stance and non-rubbing issues you desire.
Don't go with a bigger tire in front than a 215/45/17 and you will be fine with a 4.5 BS.

 

Soaring is right on. Mustang Depot has a chart made up of examples from various owners that list wheel size BS and tire sizes. Its in the wheel section by year of car.