How fast can a stock leaf rearend be?

I've seen some of you guys mentioning tire sizes like 10" or 10.5" wide slicks on pictures of Dusters. Do you know the largest width of rear tire that fits under a 71 Duster with no susp mods?
Sorry, didn't mean to hijack this thread !!! I will put this on a separate post.
Well at the time i had the 7" inch rims it was all i had. I stepped up to a Centerline 8.5" rim with 4 7/8 backspacing (look at avatar) and it fits like a glove. Leaf springs in stock location etc.