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

How much slip yolk exposed past the dust seal is too much? Experimented by sliding the axle back and with the weight of the car on the suspension it normally shows about 1/2 inch of the yolk. If I move it back say 3/4 of an inch like I need to then I'm showing 1 1/4 inches of yolk past the dust shield. I feel like that is probably too much.