Widest rim and tire for full mini tub to frame

On my '71 GT there's 12" from the quarter lip to the stock wheel tub, and a little over 2.5" from the wheel tub to the outside of the frame. That distance changes over the length of the wheel tub but that's what it is in the middle near the axle. So, call it 14.5" from quarter lip to frame with a minitub and 3" spring relocation.

You'll need about a 1/2" of clearance on each side to keep the tires from rubbing, so that gives you about 13.5" of space to work with. A 335mm tire has a section width that's 13.5" wide, so I'd say that's about the max. And since the outside to outside width on a rim is an inch wider than the advertised width I'd say a 12" wide rim would be about as much as I would run. i suppose technically a 12.5" wide rim would work but the sidewall would have to be pretty much even with the rim lip for clearance.

And the usual disclaimer that all of these cars are a little different, a 1/4" from one car to another isn't unusual. Heck sometimes they're off by as much as quarter inch from one side to the other on the same car, you have to measure. My measurement also included a stock quarter lip, there's usually a good 1/2" you can gain there by cutting it back. Depending on the quarter lip trimming, ride height, and the particular car a 345 might be possible, that tire has a section width around 14". A 335 should be a fairly safe bet, the 345 would come down to the individual car and it's particular set up.

I know @72BBSwinger has 335's on his 70+ Dart, and I seem to recall him saying he had a little room left on his car.