Also, please be certain to press the new one on as far as you can get it to go. You want very little clearance between the gear and the thrust plate...something like .001" is great and the thrust plate ought to have a ridge to ensure this occurs. If you leave too much clearance it will allow your cam gear to run in/out at idle. It sets up a sort of bouncing motion that steadies out somewhere above 1000 rpm and that motion will alter your distributor timing enough to cause a rougher than normal idle.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end