Prop Shaft Extension

Hi,
I need to extend the prop shaft some 4" to fit my project.
I talked to the machine shop guy, and he said something about a rubber tub or something inside these old Mopar shafts, and suggested me to have him make a new one.
I´d rather not, since I want the look of an original shaft.
How do you guys do this? Is there a good way to extend the old shaft?
Thanks, Stefan