There are a lot of places it could have a vacum leak. Lets get this truck running, first open the mixture another 1/2 turn, it won't hurt anything the 1 1/2 turns is just a base line setting. You probably do have a vacum leak, On the intake manifold there is a vacum fitting for the windshield wipers, it is under the carb, make sure it is not leaking.Check the hose to the wipers, maybe even plug the fitting till we get thinks running right. The gaskets on the intake, you said you put new ones on, did you use the manifold allignment rings( small metal ring that goes between the manifold and the head), these rings must be staight are it will cause a vacum leak, ASK ME HOW I KNOW. Also check the vacum advance on the distributor, it may be leaking, just unhook from carb and temporarily plug the line.
Good luck, keep us informed on the outcome.
