Driveshaft that length regardless of material or size is going to have a very low critical speed. I have lost a driveshaft at a relatively low speed at Bonneville and it is not fun. Broke the back off the motor, the trans and the differential as well as doubled the size of the tunnel and took out the seat belt mounts. Lucky didn't break my Acura as well. Be sure to talk to an expert and a two piece is likely to be mandatory. (unless you are running a very low speed diesel....)