IMO i wouldn't use that xe274 cam with stock manifolds, and definitely wouldn't use the 250@.50" with manifolds. that being said losing about .25 of a second with manifolds vs the headers does make some sense. with the "250" cam put the headers back on. you need the separation of the exhaust pulses plus the have more volume. i also i would take a very serious look at fuel delivery. dumb things can happen between the tank and carb; cracked hose, pump push rod, inadequate pump, etc. my experience with those 800 carbs has been they are more likely to be rich than lean as they come out of the box.