I have 56 Ford with a 1972 429. The 72 heads are junk, so I have gotten some 69 heads and had them rebuilt. I am getting some Comp Cams Magnum Roller Rockers and know I will need guideplates and studs. I am not sure which size pushrods to use 5/16 or 3/8. Did Ford switch sizes during a certain year? Why are some 3/8 and others 5/16? HELP!!!!!