I have a stock 79 4x4 with mismatched wheels, I already have the tires which are 31x10.5. I also have rims in mind but i dont know what back spacing i will need or what I can get away with. The rims ar 15x8 6x5.5 and have a 3.75 back spacing but I am unsure if they will work or not.

 

I forget the answer to this but the 3.75" sounds familiar.
Best bet would be to pull one of the working rims and measure existing backspace.
Front axle wheel to disk brake interference is where the backspace is the biggest issue so if mismatched, check one of those.