Three spring relocation questions:

The 1/2" offset spring kit allows for just as much tire as the full 3" relocation if you're not going to do the mini-tub. On a '69 Dart about the widest tire you can run with the stock spring locations and no quarter lip rolling or trimming is a 245. Maybe a 255, but that's more likely on a '70+ Dart. The tolerances on the later cars allowed a little more room.

The stock tubs by themselves are borderline for a 275, so unless you minitub it that's about the best you can do. A 1/2" spring offset kit will let you do that. On the bright side, the 1/2" offset is much easier, the hangers and shackles just bolt on. Just have to move the perches. On a '69 the 275's will still be tight fit. But if you're not going to do the mini-tub the wheel tubs are the limiting factor anyway.