You can put the Shaft in the freezer to shrink it & not heat the Drum as much.
CK Performance used to have 4340 or 300M Billet Input Shafts, not sure if He does anymore tho'.
You can check with Lonnie at Extreme Automatics. He may have them or know where to get one.
Extreme Automatics Where winning is easy
GM 700R4 / 4L60 TRANSMISSION INPUT SHAFT - BILLET 300M HARDENED STEEL | eBay
700R4/4L60E 300M INPUT SHAFT
Transmission shafts - High performance transmission - transmission
Have to make sure what Yr. model these fit.