I'd be a little concerned about not having enough interfacing material in the shaft now, and it wearing out more quickly. You said you were and inch too short, and you extended the shaft, so I assume you extended it an inch, but how much of the shaft is still inside the "slip/ shear joint". I don't know for sure but I would think it should be "ok" and at worst case just wear out faster than the right part, ie you had 5 inches of one shaft inside the other, and now you have 4, but if you went from having 1.5 inches before your mod, and now you only have .5" I wouldn't drive it because in my mind it wouldn't be safe.

Sean