Larger tires and wheels back spacing help

[1963chev]

Thanks Jimmy. I have done so much customization like 72 rear end and 71 front end, lowered 2/3, ps, etc that I can't really make comparisons to stock trucks. What I need to find out is how to match the back spacing of what I have on the truck now. I just pulled one of the 15x7 rims off, measured, and found that with the rim face up, it measures 4 inches from the floor to the bolt holes. With the rim face down, it measures 3 inches from the floor to the bolt holes. So, does anyone know what back spacing that is? If I go to 16x8, what back spacing should I get to match what I have now? Thanks for any help