I have a 67' cpe with a mildly built 289. Engine mods are stock bottom end (as far as I know), GT-40p heads, RPM performer intake Edelbrock carb 600 cfm (I think), 289 hi-po exhaust manifold with dual 2 inch exhaust, and comp cams 268. I always run 93 octane. The car has a C-4 with very high rearend gears, like 270. I'm having a lot of trouble with it missing. I've had my mechanic adjust the carb. I'm hoping changing the plugs will help. What plugs would work well for this combo? Thanks.......