Yeah that’s not gonna work with that backspace. A 68-70 B rear is significantly wider than an 8.25”. The 8.25” is 56-3/4” drum to drum, the 68-70 B is 60-1/8”.
If you want 15x8’s on a 68-70 B-body 8 3/4 in a Duster you’ll need 5.75” to 6” of backspace with the perches at stock A body width (43” c-c). If you do the 1/2” spring offset then you can increase that to about 6.5” of backspace. I run 18x10’s with 7” of backspace on my 68-70 B-body 8 3/4 in a Duster with the perches at 42” c-c (1/2” offset). The 15’s will take less backspace than that because of the larger sidewall bulge.