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Welcome from CT!

Excellent find. With the 4 speed and the A- code 3.50:1 Trac-loc rear axle, you should have 31 spline axles, the 1330 u-joints and possibly an N-case rear end. It's about the best of the best in '73.

Looking forward to seeing your progress.
Thanks! When I looked up the "A" axle code I thought it said 3.50:1 open rear. Am I missing something? I'd love for it to be the Trac-loc...
Ooops. my bad. The A is an open, but the 31 spline goodies should still apply.
I might be wrong but I think only R codes and boss got the 31 spline. Of course the BB cars also. The end of the axle will tell you without pulling. Google the difference I forget, lol.

 
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Hemikiller

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I might be wrong but I think only R codes and boss got the 31 spline. Of course the BB cars also. The end of the axle will tell you without pulling. Google the difference I forget, lol.
The 71-73 351 cars with 3.50 or numerically higher rear gear all received the 31 spline 9" and the larger 1330 U-joints and matching driveshaft.

Typically the 31 spline axles will have a large center countersink, with two smaller ones on either side. The 28 splines have a recessed "double D" shape int he axle end.

 

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