I went through the same drama. I want to say mine was 67” and I had one shop say flat out NO to making it a one piece and I had the machine shop for levines auto parts price it out both ways. They said it was just over what they feel comfortable doing for length but they understood that it was substantially more $$$ and I explained to him that I already did all the calculations and I understand its right on the edge of “safe”. They said we will build it for you the way you want it but there is NO guarantee on it. I said ok do it up. Ive had them build 2 shafts for my racecar so I wasn’t worried. I did learn a lot about how driveshafts work though.
Just fyi a 90hp truck can explode a driveshaft driving down the highway on cruise control. Nothing to do with horsepower but its all related to driveshaft RPM. Imagine a truck with a 4.56 and say a 28” tire AND OVERDRIVE. Now cruise at 80mph and that engine might be singing but ok but the driveshaft is spinning its heart out! NO driveshaft is perfectly round and no driveshaft is welded perfectly centered on a yoke. There is a acceptable run out and thats where balance comes in. Now when you spin it fast enough to exaggerate a imbalance the tube basically turns into a jump rope and then it self destructs and stuff goes bad quick. Also the longer the driveshaft the harder it beats up the joints.
I knew none of this until i really really researched it. Aluminum is a good option because you can go longggg and not beat up the joints. Hence all the new trucks have long 1 piece aluminum shafts in like 5”.
Use this site in the future. Works well if you input the correct info. Remember the larger the steel shaft diameter the heavier it is and shorter is needs to be if its a 1 piece.
Driveshaft RPM CALCULATOR 
