72bluNblu,, My ride height from the top lip of the rim to the bottom lip of the fender is 5 3/4" with no rubbing issue in cornering or swags.. I have this on a 69 dart not a duster...
Dart vs Duster doesn't matter that much for front tire clearance, especially if you're talking about the fender lip. I ran Duster fenders before, I'm running '71 Dart fenders now on the same car (demon tribute). My current front tires are 275/35/18's and the outer face of the wheel is in the same place as my 225/60/15's were. On purpose actually, because I knew I couldn't run any further out and still clear. But I run my ride height so that the tops of the tires and the bottom of the fender lip are at about the same height, tire might actually be a little higher.
From your measurements it sounds like your fender lips are a good 1/2" above the top of the tires. You won't be able to lower the car any and not have rubbing. With the right conditions you could probably still get rubbing, it would just take some suspension compression while turning. Like hard cornering while braking. I suppose on a cruiser that's ok, but that's not how I set my clearances.
