I have a 68 coupe w/ 289. Built the engine about 20 years ago. Cams trashed and is currently at machine shop getting full long block rebuild. Girlfriends dad at the time had a machine shop. We put flat top 9.8:1 hyperuetectic pistons, big cam, roller tip rockers, 1.94/1.60 valves, cast heads ported and polished, performer rpm intake, 600 cfm holley double pumper. Replacing cam with Comp 262xe. The valves r too big to machine hardened seats. It was suggested to maybe run a lead additive in the gas or even just spring for leaded race fuel, maybe mixing in intervals. Has anyone else done this? Is it advised? I never ran lead before but engine only lasted about 10k miles before I think I over reved it. Please advise.