I always figured slide the yoke in all the way till it bottoms out either on the shaft or the rear seal and pull it back out a half inch or so,as long as the rear seal has a nice smooth area to ride on,all you have to do is make shure the yoke doesn't bottom out on the trans shaft or seal when the rear suspension bottoms out,generally with these cars anyway the yoke moves very little,If you're having a driveshaft made they'll generally want you to measure from the tip of the trans output shaft to the flage of the rearend yoke which is bassically center of the U-joint,from my experience using that method ususlly puts the yoke the same place,about a half inch from bottom,Hope that helps!KillerDave