Vintiques 15x10 rallies center bore fitment issues

[82Cecil]

Hey folks attempting to stuff 315/60/15s insude a stock 1982 SB rear (4" drop) wheel well. The truck for now has a stock 10 bolt axle. wheel has 5" back spacing (-12mm offset). the unexpected issue is the axle hub that passes thru the drum is measuring 3 1/16". the 42-500505 Vintiques are a 3" center bore per the manufacturer.

There's a lot of folks running rallies, the hub diameter never came up with all the concern about dialing in a backspacing that fits max tire in a stock wheel well the the 2.5/4" drop combo.

What's the formula/recipe to fit the 315/60/15 on a 10" wheel ?

Thanks for your help