I'll bet you have an aftermarket gear, and if you do it will be made in China. None of the aftermarket products that are sold by the street rod vendors fit like the OEM parts. GM required the suppliers of assemble parts and that includes every part on a GM automobile to be within specs and if the part didn't fit, GM sent it back to the vendor. That doesn't happen to the Chinese parts because we are so happy to get something realitivy close, were happy.
Your gear should not need heating to slide on, it should go on with little effort. Like I said, if you got this gear from Summit, Jegs, Crane, Comp, etc. it will be made in China.
Sorry for the rant.............been blocking rear quarters on my '58 all day and nothing went right. I think I'll go have a beer.