Not sure if you were exaggerating on the length remaining of the driveshaft, or not. But no, a 6" driveshaft is just not going to work unless you're going to lower the suspension, run a high-pinion rear end, and ditch the slip shaft.
The double-cardan yoke alone is longer than 6" by the time you weld it to something. Then the slip-yoke is longer than 6" long by the time it's welded to something. Even if you used just enough tube to connect the dots, you'd have a 12" to 18" shaft most likely.
Just guessing off the top of my head of course. But it's not going to work.
Now, if that was just an over-statement of the transmissions overall length (I have no idea how long it is myself) then maybe there is still room. But once you get there, you still have to make allowances for angles and such.
Up to you of course, but that free-because-you-have-it transmission might turn out to be more expensive than buying another one and selling this one to help offset the cost.
Good luck. Whichever way you go, we want lots of pics!
Paul