[Coley]

I'm pretty sure you need a vacuum line running down to the a modulator which would be located on the passenger side (forward) on the transmission. It is round looking diaphragm device with a vacuum port on it.
I'm not sure what port it connects to up on the engine but usually, on the stock intake there is a vacuum port (spiggot) on the manifold...or maybe on the carb?...that the rubber vacuum line runs down from to the transmission (modulator) port.
Visually Check to see if you have the modulator diaphragm unit on the trans...it would be sticking out towards the side of the truck (at right angles to the driveline)....is there anything on its vacuum port/spiggot? ...any quick pics of the passenger side of the trans available?...and maybe of the back of the intake?
Regardless, maybe some of the other guys more familiar with the TH400 can weigh in here as well.
(note: I also recommend 'googling' GM TH400 transmission modulator vacuum...this will help with some of the understanding)
Keep us posted.
Coley