The 7 1/4 axle was the standard axle behind slant 6 and 318 cars. Its a decent unit with great reliability. Yeah, they break, but I have seen 8 1/4 axle shafts break at the hub without being abused. It really comes down to how the axle is treated.
Even using a 3 or 4 speed trans doesnt spell instant death. These axles came behind 318 3 speed cars too.
What kills these and other axles ? Abuse and neglect. Rev it up and dump the clutch? Peel out alot? Buy some good shoes. Also, and this also applies to early transmission failure...When spinning the tires on loose surfaces,(Water, dirt, gravel) LIFT off the gas when approaching an area with better traction. Its the sudden shock loads that cause the greatest damage.