I have fabricated driveshafts considerably longer than the ways on my lathe by simply removing the tailstock and supporting the shaft in the middle with a steady rest. The tricky part is getting the shaft tack welded back to the U joint yoke or stub spline with minimum runout before doing a full weld pass, getting the spline and U joint clocked properly, and then balancing the whole assembly. For minor driveshaft balance problems I use a big hose clamp and a 1 ounce wheel weight with the vehicle running at highway speed on jack stands, and move the weight around until the shake goes away. Then I weld a flat washer the same weight onto the sweet spot I found with the hose clamp.
Jerry
