For ball joint clearance calculations, I hate dealing with offset, since there is another variable - wheel rim width.
Calculate the backspacing of the new wheels, and make sure its less than 5". 4-5" of backspacing is pretty much ideal. Just keep in mind if you get super wide tires on a narrow rim, you could also run into the ball joint, as it is also a variable.
viewtopic.php?f=14&t=26The option you mentioned with an 8" rim and 20mm offset comes out to ~ 4.8 in of backspacing (check my math). If you're running a 265 or 285 width you should be ok, but you're getting close. Get up into the 305 and you might have issues. Depends on the margin of error that you're comfortable with making a decision on.