Well, I could be mistaken but I believe the axle to U joint centerline is longer on the 8 1/4 (about 1 1/2" to 2"). If you used the same DS, assuming it was correct for the 7 1/4 and you installed it with the rear end hanging, you may have issues soon enough. If the slip yoke is too far into the trans and you load the rear suspension real hard it could blow out a U joint, or tear up the tailshaft on the trans.
