I'm curious to know how the thing was running overall before you started having these issues. How's the fuel economy and performance? The general consensus amongst the tech community hereabouts is that particular list number carb is calibrated too fat to be ideal for most sv applications, especially the 266 and 304 engines.
Often when an engine has trouble maintaining idle all of a sudden, its due to a blocked passage inside the metering block. It doesn't take much grit sucked in with raw fuel to cause a problem. Are you running a filter both upstream and downstream of the fuel pump?
You can remove the idle mixture screws and squirt some carb cleaner into the holes chased by a quick blast or two of compressed air. Replace the screws and adjust each equally for a lean best idle at normal operating temperature.
Where are you sourcing the power for the electric choke? Hopefully not from the coil + terminal. It needs to be from a key switched on source. A choke that is not able to open fully and stay open once the engine has warmed up can be problematic. You May want to verify your choke function.
A vacuum leak will also negatively affect idle quality. If this truck has vacuum assist brakes, a ruptured diaphragm inside the pb booster can create a vacuum leak that is most problematic when the brakes are applied. To rule this possibility out, you can disconnect the hose at the pb booster and plug it. If the engine is suddenly able to idle in gear without stalling when the brakes are applied, and then stumbles again once the hose is reattached, you've found the source of your leak.
