I think you're going to need a Duster for that size tire...
Nope. Just a 1/2" spring offset like the OP is planning
I run that size Nitto Drag Radial on my '68 Barracuda convertible on an 8 inch rim. I forget the back spacing. The tires don't rub anywhere, are rated to 186mph and hook up very well on the street. They don't hook as well as a dedicated slick on a prepared track though.
I do have 3200lbs SS springs, but they fit the same as a stock spring as far as spring size etc. The car weighs 3700lbs without me in it, so it doesn't sit to high in the back.
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Barracuda is a completely different animal than a Dart. About an extra 1" to the quarters compared to a Dart. What will fit a Barracuda, Duster/Demon/Dart Sport is
completely different than what will fit on a Dart.
Same for the SS spring thing. On a Duster, Demon, Dart Sport or Barracuda the SS springs raise the car enough that the quarter opening is higher than the top of the tire. When that happens even if the tire wouldn't fit inside the wheel well you don't always get rubbing, because the wheel wells are so high above the tire it doesn't matter how much clearance to the inside wall of the quarter there is.
On a Dart the wheel opening is so low it's darn near impossible to raise the car so the top wheel opening is above the top of the tire.
Not without either doing the 68' Hemi car quarter cuts or a mini tub and spring relocation. A 255 will squeeze in there with the right offset rim and a inner fender roll. I run a 245 in the rear but I have a rear sway bar to stop some rub'n. I won't mention the hill billy air shocks to jack the *** up enough to get them to clear.
I have a 69 Dart. 67-up Barracudas, Dusters, and Dart Sports have way more room due to the open quarter skins vs the Darts truncated skin.
Again, nope. With a 1/2" spring offset there's room for a 275/60/15 on a Dart. And I have no idea what you mean about the "open quarter skin" vs a "truncated skin". In the words of Inigo Montoya “You keep using that word. I do not think it means what you think it means.” The wheel opening is lower on the Dart's, but that's not what you actually said there.
On a Dart there should be ~12" from the inner wall of the rear wheel well to the stock quarter lip (unmodified by cutting or rolling). That does vary a little bit due to body tolerances. If a 1/2" spring offset is used, the limiting factor on the tire size is the wheel well, not the spring location. That means if you leave a 1/2" for clearance on the wheel well and a 1/2" of clearance on the quarter lip you should have room for a tire that's 11" wide at it's widest point. Well, the section width of a 275/60/15 on an 8" wide rim is 11". Which means if you center the rim in the wheel well you should be able to run a 275/60/15 on a Dart. Body tolerances can be a little tighter on the pre-'70 cars, but, if you're willing to modify the quarter lip by either trimming or rolling it, you can get as much as another 1/2" of space. In most cases that will get you enough room for the 275's even if your car has tighter tolerances than the initial 12" measurement.
And it's been done before, you can look at these threads for guys that are running 275's on their Dart's with the 1/2" spring offset. It's tight, but especially if you're willing to trim or roll the quarter lip it's possible.
275/50/15 Nitto NT555R no tubs or rubs
Max tire size - 68 dart
275's W/offset kit? 69 Dart
15x8 4 1/2 BS 275/60 stock spring location
