3" steel is a minimum. Depending on the length, they may want to go to 3 1/2". Especially with a manual trans.

Local shops are sort of okay for street rods and drivers, but none of the local places I have can spin them fast enough to get the balance right. And they also have trouble reading a dial indicator. Guys are used to building stuff for dump trucks, not race cars the go 8000RPM.

After a while, I just stopped trying to get it right from those guys.

Considering what usually happens when a shaft fails, I would not cheap out in this area.