Wheel back spacing (HELP)

[66farmer]

Time is running short for me. I need to order my wheels and I don't have the bed or the front fenders on as of yet. So I need some help from you guys.
My truck is a '66 short step-side 4.5" drop front and 6" in the rear. Stock rear axle width 5" on 5" bolt pattern, the front is 78 disc brake transplant. The rims I'm going to buy are Boss 338 18" front and rear. I want to fill the wheel well but I don't want to rub. Please let me know what has worked for you guys.
If you can only comment on the front please do.

Thanks for your help