pitman problem please help!!!

[78chev]

Ok, so I am rebuilding the entire suspension/steering on my 78 c10, I installed the new p/s pump, hoses and gearbox, bled the p/s with the pitman arm disconnected so that I didn't have to lift the front of the truck. Then I went to connect the pitman arm to the gearbox and found that the pitman shaft has 3 flat spots on the spline shaft versus 4 on the arm so they will not fit together. The gearbox is a reman from autozone part # 7512 and the pitman arm is the one that was on the truck. Autozone is ordering a gearbox from a different manufacturer to compare and see if the first one was just a mistake. My question is does anybody know if the pitman arm on my truck is the original, and if not, what is the part# for the one w/ 3 flat spots. Thanks for any help.