I have 275-35-18s on my 70 Duster and I had to only push the support strut out on the front and replace the hex bolt with a pan head for the fender brace in the front. The car has the MP offset shackle kit in the rear and that's it...and I have a mile of room to the springs now.

I think the 70-76 Duster/Demon/Dart Sport is the car that's easiest to get there. I doubt you'd even have issues at high speeds, given that Vipers are much shorter. You'll need to get the caster way up (I have +7) and my car loves going straight at high speeds. More caster is one of the things that modern cars have much more of than an older stock vehicle.