This picture is on the underside of my hood. It's for a fuel-i 302. Hopefully one of the 351 guys will jump in and help. Anyhow, I think you have a vacuum tree on your manifold, and it's prolly gonna be close to your carb. If my memory serves me correctly, it will be between the carb and the firewall, but I'm not 100% on that.
I'll try and find a picture of a vacuum tree for you. Maybe you can go to the thread of motor pics and look through there. Try goin here ---> click me
Ok, let's start over. Did someone make modifications to this truck before you bought it? Second, have you traced out the rest of your vacuum lines? Were is the other line from the coffee can connected to? And you are asking if this does what to the tranny?
As far as which place to mount it on the tree, don't worry. It's going to be the same amount of vacuum no matter where it hooks up on the tree. The different sizes on the tree are for different size lines.
Ok, you have a TV control rod, which is connected to the carburetor. If you have done something to keep the 'butterfly valve' open, you may have to adjust the TV control rod to compensate. There are 2 places to make the adjusment. You can adjust it at the carb or at the tranny. I suggest that you adjust it at the carb, since that is where the problem started. You may have loosened it by accident. Second, you need to look into the cause of the 'butterfly valve' being stuck. Has the carb been rebuilt? How dirty is it? Third, trace out the other lines coming from the vacuum tree. The line that was hanging may be a 2 piece line with a union or whatever in the middle. The other end may be hangin around there also. Good luck, and I hope that this solves it.:thumbup