Welcome, Gil. My Michelin Pilot Sports had a
measured section width of 13.25" when mounted on a 12" wheel.
The wheel to fender lip depends on ride height. The further you have the wheel/tire tucked up into the wheel well, the less room you have.
3/8" to leaf spring sounds fine to me, I've seen tighter setups. Be sure to look at the clearance from tire to inner fender. When the body rolls when entering/exiting driveways, you can get some rubbing depending on how much room you have.