Working on the '68 V-6 over the weekend, we were never able to get it running for longer than a minute. After a couple pumps of the gas pedal and choke open, it would fire up and run normal for a few seconds, then fall back to just sputtering, then rev up to fast idle again, then sputter, over and over again. Often would stall during the sputtering part. We replaced the original gas tank with an MTS plastic one the last time we worked on it and it ran that weekend without a problem. I'm thinking it's acting like there's a vacuum in the tank. Buddy thinks that we may have ignored the placement of the tubes in the tank during fuel pickup install when we did the tank. We didn't come up with these theories until after we left, so we couldn't test them. Either of those sound right, or do y'all think it's something else still?