I dont know how, but the custom axles in my 65 Plymouth ended up being the same length. The pinion is offset in the case to the passenger side 1-1 1/2 or so??

But the axle length might not be the only thing that can cause problems. The spline pitch on there axles are diff, even though the axle spline count might be the same, it stiff wont work...
Id do some homework on this one, starting with a phone call to mark Williams.