i have a 1971 duster i put welds 15x8 with 255/60 bf goodrich tires on the back i have 15x6 for the front what size tire should i use that wont rub thanks

 

It may depend on the stance you want. I would go with 70's, maybe 225. The 6" wheel will be a factor.
Check out the link for diameters, widths and cross sections. Maybe you can get a tire sales to mount a tire and check the fit with out driving on it if you commit to buying from them.

http://www.1010tires.com/tiresizecalculator.asp