71 Gearbox VS 73 ?

No.

The 73 has a larger output shaft diameter that goes with a 73-76 large spline pitman arm.

If you use a 73-76 centerlink AND 73-76 A-body idler arm AND 73-76 pitman arm you can convert your setup. You must have those 3 other pieces to complete the swap.

No performance or driving difference.

The replacement 73-76 A-body idler and pitman might be a different prices than the 68-72 replacements.