Another tell-tale sign of a 4000, your 19,500 GVW. I wouldn't give up on the Carb yet. Try to see if you can get a rebuild kit for it. I've had many V-6's and not one gave me any Carb trouble. Although many out there like to switch to a Holley. If all original, it should be a Stromberg. I've had V-6 engines from 305's to a 478. All ran great with the original Carbs.
