IMO I think you have a case of too big of carb. If that's an 850 Mighty Demon it has 1.56 venturi with a 1.75 thottle bore base plate. Essentially the biggest combination Barry Grant offered in a 4150 style carb from what I know. On the flow bench that thing will go 1000+ CFM. That carb is better suited on a 450+ cube motor. Its going to be a finicky temperamental bastard on a 408. You may also be lean as that carb should have 85/93 jets, this could be why you have a stumble.
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