The 52 rear axle will be the torque tube type, (enclosed driveline) so the only options you have without doing a rear end swap will be the current 4 speed or a 3 speed, which would require a steering column change and several other modifications.

If you're willing to go to the expense and effort of changing to an open driveline rear axle, a Muncie or Saginaw car 4 speed could be used. Either one will still have a 1:1 ratio 4th gear, so there will be no gain in road speed. A swap to a T-5 overdrive trans (5 speed) will give you more top end, and 5, not 4 gears to work with. All the above options will require changing the torque tube rear end that's currently in there.
Jerry

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