Can't find spec's on comp-cams 274H only 270,what's Duration and lift on your cam.are you using stock valve covers,rockers could be hammering on the inside of the valve-cover's.just a shot at it.500 CADDY I think I no where your going if it's indeed Mech not Hyd.I run a MECH in my 454 turn up the stareo or put on lowder mufflers.