I have a 1990 dodge w250 gas. I want to take my 8 lug 170mm off set of and down them to 15's. what would i need for an off set?

 

why?
any cost savings in rubber will cost you in fuel. not to mention the better selection for 16 inch rims.
I'd try to stick with the same off-set, or maybe a little smaller if you're planning on more narrow wheels than you have now, and a bit larger if your getting fatter tires.
Either way I wouldn't deviate from 170 too much.

but thats just me, wait for crazywolfie

 

wait for me? you pretty much covered what he asked with the info given. in order to give a more accuret answer we would need to know the size of the current 16inch tires you have and the size of the 15's you plans on using. i would also like to know why you would want to do this?

 

depending on what kind of drums you have in the rear may not allow you to install 15's, i have run into this a few times before on 3/4 ton's, but cant remember which vehicals they were

 

be easiest, I'm thining, to just do a rear end swap. Might need to change the yolk on the new rearend or on the drive shaft (probably be easier on the rear)... IF you even have to do that.
Leaf springs aren't as much a pain as coil springs to release from the frame, then there's the break fluid line and the parking break wire. pretty simple.