A buddy picked up a set of the classic industries small bolt pattern 15x7 steel wheels. He is wondering what size tires he should use with them on his 68 Dart GT? He wants to get 4 of the same size tires. Any recommendations? Thanks!

Unless he has some specific goal, basically the parameter would be a tire with same diameter as stock, and at leastr the same load carrying capacity.
Car probably came with E70-14. That's roughly a 25.5" diameter tire.

In 15" Close sizes are:
215/60R15 25.16" dia Probably the safest bet in terms of fit.
235/55R15 25.18" dia
225/60R15 25.63" dia. Check to be sure it will clear the fenders under hard turns.

Put those sizes into a search such as at Tirerack or 1001 tires and see what's available and what the specs are. there's also some brands that might get locally but are not carried by the internet sellers. But at least it gives you an idea of availability of different types of tires and their specs. Make sure to check the recomended rim width for the specific tire and size.

I would see what rim width/offset he has now & how much clearance on either side & same with tire bead width to get a close approximation as to how much "extra" rim & tire will clear the leafs/inner fender. EDIT meant to say "sidewall" bulge not bead

Thanks!

Hopefully he has 4.25” backspace in them.

If so...

safe: 215/60/15 front, 225/60/15 rear. More aggressive 245/60/15 rear