8.25 rear in 69 dart change rear wheel back space?

I just found out the 8.25 I purchased has 3.21 gears & is a sure-grip.

My car has the 7.25sbp in it now -- how will this effect the back spacing needed on the car?
a sbp rear needs roughly 4.25" on a 15x7" wheel-- I'm planning on going with a steel wheel look---

my question is what is the usual back spacing on a 15x7" bbp wheel for a 1969 dart?

I have 2 15x6.5" now & 1 15x7" bbp steelies --so I'm hoping these factory wheels will fit.

Thanks Lawrence