I have a 87 dakota that im setting up for the strip and need to know whats the widest wheel combo i can run withstock spring location and a stock width 8 1/4 rearend?what rim&tire size and backspace......

Do you know all the 1st gen Dakotas had an inset stock wheel (4" back spacing)? Even a standard offset 6" wide wheel puts the tire at the edge of the fender lip. Anything more then 7 with a 4" back spacing will put the tire at the fender edge. If you don't mind the tire hanging out, go as wide as you want, with a 4" back spacing. Gene

Ok,thank you.

I put 15x7 (maybe 15x8) american racing rims off a B body on my 90 Dakota. Everything bolted up fine, but the different offset gave the Dakota a very noticeably wider stance.

So a 15x8 1/2 with a 10 inch slick with a 5 inch backspace should sit flush with wheel lip?

I think a 10" slick on an 8 1/2" wheel with a 5" back spacing is going to put the outer edge of the slick outside the fender lip. The 1st gen Dakota came with a 15 x 6" and either a 3 1/2" or 4" back spacing, and those tires were close to the wheel lip.
I'm posting a picture of my 93 4x4 with the standard 15 x 6" wheels and a 235 75 x 15 tire. you can see how close the wheel tire fit to the box wheel lip. Gene

K,thanks for the info.

this picture is a standard 15 x 6" wheel with a 215 75 x 15 tire. There is not much left of the lip, but you can see how much the tire sticks out past the lower box sides. This is a 90 Dakota.

Last one, this is the 15 x 6" Dakota wheel shown above without the cap, notice how far forward the center is compared to the rim?

Your 15 x 8 1/2" adds another 2 1/2" to the rim width, but the additional 1" back spacing still leaves another 1 1/2" of rim width outside where this one sits, then your tire width is wider then the rim and sticks out another 3/4". Your slick on the 8 1/2" wheel with a 5 " back spacing should stick outward another 2 1/4". Gene