Carb too big?

OK, good sign in the inside being clean. /6's can get the valves to stick easily if they get gum in the guides.

Since you have not idea on condition, back up and start with a set of compression readings on the 6 cylinders. Take out all plugs, prop the throttle wide open, and then take compression readings. You can read up on how to use a compression gauge and get one as a loaner tool form many box auto parts stores.