i run a 15x8 with 4.5 inches of backspacing on my 71 duster
the tires on the back are 275/60-15
i had to run about a 3/16 spacer to get far enough away from my springs (and its way closer then what you are trying to get)
and 15x7 with 3.75 backspacing with 205/65 up front
the backspacing for my front is perfect and i like the way the side wall looks
on a 15x6 the sidewall would look more bulging, if you catch my drift
so perhaps a 15x7 with 3.25 backspacing would fit the front best?
and for the back, if you got the studs for it, just run a 1/2 inch spacer
