Back spacing help

I would go on tirerack.com enter your tire size then check specs for section size.

For example if you have a 8" rim with a 275/60/15 tire. The tire section width will be about 11.1"

That tire is about 3.1" wider than the rim. Your tire bulge would be about 1.5" on the inside. Then you may want .5" clearance from the leaf spring. Add them all together back spacing 5", tire bulge 1.5" and clearance .5" you would need at least 7" from face of drum to leaf spring.

Make sure you check both sides of car could be different side to side by 1/4" or more.