Dies when put into gear

[mvfd70]

I've got a 78 Silverado with a 400 SBC that is all original. I put a new fuel pump and some new fuel lines on it today to fix a leak. It has been sitting for a couple of years. I had to jump start it today and it started pretty good and ran great. Idled no problem and revved with no problems in park. I let it warm up for a couple of minutes then put it in gear. As soon as it got a little load on it from the transmission it sputtered then died. And of course since it's been sitting the battery is dead and I had alredy moved my other truck that I jumped it with so I didn't feel like messing with it anymore. I'm thinking that this is an issue with the carb. Do you guys think i'm correct with my assumption? This truck was doing the same thing before I parked it a couple of years ago.