Clueless on wheel backspacing - Need Help

[JWPOORE]

I would like to upgrade my wheels on my 1970 Chevy Blazer 4x4, but I am clueless on what the right backspacing I need for it.

I currently have this:
33x12.50x15 tires, which are what I want to keep on it.
15x10 rims - clueless on it's backspace, 6 lug/5.5

There is a couple of different new rims I have seen for sale that I like, but I'm not sure if the backspacing is correct or will work for my truck:

15x10 with 4.625 inch backspacing
15x8 with 3.625 inch backspacing

Thanks for any help on this matter and suggestions on if either size will work for me or if I need to look for ones with different backspacing. I have tried to research this, but keep coming up more confused.