The factory yoke to seal clearance is WAY too generous in every application I've seen. There is often more than 1.5" of clearance and 3/4" is plenty for a leaf sprung car.
I'd recommend installing the old driveshaft and checking yoke to seal clearance before you cut anything. If there is 3/4" or more, it will be fine. As long as it doesn't bottom out, the more spline engagement, the better.
