Tom Woods or Adams Driveshaft will build you whatever custom shaft you want. I have used Tom Woods a couple times with good luck. Tell them what joints/splines needs to go on each end, the length, and what kind of slip splines you want. They walk you through it and will give advice on what will work best too.
FYI, I had a local shop give an estimate to modify my old rear shaft to work with the 241 case and Tom Woods was cheaper to build a custom shaft.