[Agv63]

The 63-64 coarse spline axle shafts is not bad, mine held up with occasional hard driving with 31" tyre and pontiac 455 power for about 3 yrs. Lighting up those 31s is easy, went the 72 5 lug axles after that as i went discs as well as bigger tyre to help bring cruising rpm down, i can tell you that the seal for shafts is 0.040 smaller on the outside dia, didnt notice on bearings being different.

With a inline six or small'ish v8 and normal sized wheels i dont think there will be issues, depends on your engine and driving style and use.

Cheers.
Adriaan.