well stock backspacing is around 4.5"
if my numbers are right if you did a 8 in wide rim thats 1 inch over stock(.5 on each side)
3.75"bs would be .75" over stock.
.5 + .75 = .8" or almost close to a inch more sticking out... or something like that.
if you want to get an idea you can search for pics of different BS mounted up