There was a running change in the spline count on the axles in '65. For the most part, '63/'64 are 17 spline, like their '55-'62 HO33 predessor. The '65-'76 12 bolt is 30 spline, though I have heard of early '65 Truck 12 bolts wiht the earlier 17 spline axle.
As far seals, I'm not sure where you've checked, but Autozone has them listed for $20 apeice, part #84775, which is about the same price as the seals for the '55-'62 axle.
