just got me a '67 Barracuda

It certainly sounds like a vacuum leak but as Redfish stated " when you set the timing do you remove and plug the vacuum line going to the distributor" The mechanical advance will pull in about 20 degree of timing by itself, your intial timing should probably be about 7.5-10 giving you around 30 degrees total at 3000 rpm. These are just ball park numbers to start from but the car should run without any problems with these. When you changed the distributor did you check the vacuum advance, the diaphragm could be torn-vacuum leak. Also rather than clamping off different hoses, remove them at the source and plug the port ruling out a bad hose. Good luck.