measuring for wheel and tire

Take stick and lay it across the rim (laying on the rim lip/edge) then take ruler/tape measure and measure the distance between the hub and the stick. That is your backspacing of the rim.

To do the wheel well its the same basically. A stick from the axle hub to the top of the wheel well then measure distance from stick to the leaf spring (should be the closest point to the axle hub unless you have changed their location) or the wheel well (whichever is the shortest distance). Write that number down, this gives you the backspacing. Then measure from leaf spring/inner wheel well to the outer wheel well and that will tell you how wide of a tire you can have in your wheel well.