I am putting a Competition Cams 4 X 4 extrem cam in my new to me 350, also having heads rebuilt, new lifters and in the info with the cam they recommend putting a new distributor drive gear in. I have found Bronze, iron, and carbon ultra poly composite, also iron. Any opinion on what is better. Also it appears that you can get them in either a .491 shaft or .500. Is .491 stock size for GM HEI???????