Rear end location wrong? IM STUMPED!!!

[72bluNblu]

There are crager (sp) 14" rims w/ 225/60r14 front and 245/60r14 back tires on now. I'm hoping I can go to a 15 or 16 diameter rim with a 225/50r15 or 16 on the front and 255/50r15 or16 on the back with a 7in wide rim. My thought is less bulge in the tire may help with clearance and I'm not a big fan of the taller tires like my pops was.
I need to figure out the backspacing on the rims and plus or minus them correct?

Based on your measurements, your current tires would fit just fine with no rolling if you had an extra 1/2" of backspace.

255's have been done on some 70+ Darts, but, they're super tight. Now, if you roll the quarter lips AND buy a new set of wheels with more backspace, then it might be possible. But, with the tolerance stack you have going on, I think your better bet would be to increase the backspace if you want to buy new wheels, but keep with the 245's. You're not gonna find a set of 255/50/15's or 16's either. That's just not a size anybody makes in a tire you want to run.