Perhaps check your coil also. You didn't state if you have points or electronic ignition. If electronic try a different module. If points check your advances( mech and vacuum) along with the points themselves( point float from weak spring). Cracks in the dist cap, bad condenser.How old is the carb? Has it sat with old fuel in it?( varnish in the jets)Loose throttle plate? Intake leaks? 351 heads have the bigger ports everything match there? (intake to heads etc).How's the valve springs? Did you check your valve to piston clearances for your cam.( bent valves/valve interference)Good Luck