I just did this in my wife's 56 Nomad, but moved the engine forward 7/8" for the TPI/HEI engine, so it depends on if you are stock or forward.
The right way to do this is with a tape measure. If you have the front yoke - push all the way in then back it out about 1". Measure from the centerline of the u-joint pocket on the yoke to the same spot on the rear end. If you dont have the front yoke, measure from the end of the tranny output shaft to the center of the rear end ujoint pocket. The drive line shop will do the rest for you as long as they know how you measured. I just had mine re-tubed to add the 7/8" inch and cost me $175 with balancing.
I neglected an important thing - do the measurement with then car on all 4 wheels so the suspension is compressed at normal ride height. I use some old 8" wheels laying flat on the floor and set the the tires on them instead of jack stands, but stands under the rear axle will be the same effect. The rear end will be farther away if it's hanging on the springs with jack stands on the frame rails. The purpose of the 1" backout is to account for the rear end going up and down with travel which pushes the driveline in and out of the tranny. Remember, measure twice and cut once!
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