73+ disc brake backspacing

It does widen the track, how much depends on what brakes you had before but it can be almost a 1/2” per side.

I ran 225/60/15’s on 15x7’s with 4.25” of backspace on my ‘74 Duster with the 73+ disk brakes, that worked fine. The equivalent backspace on a 15x6 would only be 3.75”, that would put the centerline of the wheel in the same place as a 15x7 with 4.25” of backspace. But I assume you’re talking about a ‘64?

You can run more backspace if you want the wheels more tucked in or need the clearance on your ‘64, but with 15” wheels you generally don’t want to run more than 4.5” of backspace because you run into brake clearance issues. Some wheels have better brake clearance than others, but that’s the general rule.