It's very simple, and as you have a 41 truck on a s10 frame, the fender is probably wider than the s10 box was... Im guessing, I am building a 51 chevy on an s10 frame and I fit 15x10 Rallyes with 4" backspace with 15" wide x 31" tall M/T's.
The stock s10 wheels fit waaayyy to far in on my truck. I dont have pictures, but I would have had to run a spacer for sure, or cut the inside box, but again, they were way to far in and looked silly.
You might be able to cut your inside box panel to get a big tire all the way to the leaf spring like I did. You could also buy a wheel spacer to go between the drum and rim if you need it out further than a wheel you can find.
backspace is measured from the wheel bolt face to the back of the wheel. you can find exactly what you need by centering something 10" wide in your wheel well, (like a 10" block of wood, for visual aid) between the inside fender or leaf spring and outside fender, and then measuring to the back of that piece and the drum face. that will center your rim in the wheel well and will allow you the biggest tire, or if you want it closer to the outside fender just use your guide and put it where you want and measure from the back to drum face.
Then you can measure for you biggest size tire by keeping atleast a 1/4" distance from fender edges.
Theres lots of diagrams showing this if you google.
