How do I figure this out??

[phils88gmc]

Ok, I have this 88 GMC I got on a trade. It is lowered front and rear, I would like to put some nice wheels and tires on it and can't figure out the backspacing on the wheels, do to it being lowered. I have no clue in the world how much it is lowered. Does anyone know how to figure something like this out. I would like to go 20" in rear and 19" in front.

Now some one flipped the axle on top of the leaf springs and c-notched the frame. I can't tell if they put lowering leaf springs in the rear or not. There are no blocks in the rear either

The front has tubular lower control arms and i cant tell if the spindle and springs are aftermarket or not.

I just want to be able to order some wheels and tires and have the correct backspacing ready.

Here is a pic of it now....