What will my 68 Dart 408 run in 1/4 mile?

Not sure if it’s been mentioned, but you can drill a new hole in each spring pad on the rear. Drill them 1/2 inch forward of the holes there to move the rear back, and if all is well take it back out and plug up the original holes.
Half inch and you might get away without changing the driveshaft.
Those tires look great on your car