Output shafts from th350c and th350 can be interchanged. I'd just swap the output shaft with the length you need and be done. Takes about an hour with the transmission on a bench. I remove everything and set it on a clean foam sheet in the order I remove it, except for the clutch packs, I slide those over a horizontal tube in both the same order I remove them, AND the same orientation & clocking.
Not all longbeds came with 2 piece shafts either, I still have the driveshaft out of my '84 GMC c1500 hanging in my parents garage..its a one piece driveshaft for a TH350c w/ a 9" tail housing.