Arlie,
To my experince The rear end in you car should be an 8 inch vice a 9 inch. 390 or 428 cars mostly have 9 inch rear ends. Some "J" code cars may have a lower (3.50 to 4.11 ratio) which may be a nine inch a posi and have 31 splin axls. All in how the buyer checked the option sheet. Any thing could happen.
To my knowledge all 2.79 rear ends of the 8 inch variety are 28 spline. Many 9 inch open rear ends and some posi 9 inch are also 28 spline. The lower ration 9 inch, 4 pinion, traction lock (posi) rear ends mostly or all have 31 spline axels. Hope this helps.
Rob