I believe you need a looser converter as stated. That converter you are using is for a truck or a passenger car motor. All 340's from the factory came with a converter that would brake stall at 2800-3000. A advertised 3500 will stall at that with a mild 340 motor as yours. I have a 5500 and I drove the car on the street and raced it. 10 second car through the exhaust. full interior spare in the trunk 3650 lbs. Distributor is locked timing at 34. MSD billet with a "digital 6" , I ran pump gas on the street for cruise night.
