Diff/backspace confusion

[USA69Z]

Guys , i have a 70 coil rear stepside truck..I am replacing the diff with a local GM Holden (australian) 10 bolt unit as you just cant source US stuff here easily. The New diff measures 61.5 inches drum to drum and the rims i ordered before the old diff let go are 15 x10 with 5.5 backspace.. My question is will these rims still fit with this replacement diff?

I think the original 70 model width is 62.25''

thanx , Gav Downunder

[oldspowered67C10]

I have that backspace on my 67(original narrow rear end) and it fits pretty good, 5.25 bs would have fit a bit better but thats with 20x10s and 295/35-20s tucked up inside the wheel well.