That looks like a Carter AFB Edelbrock/Weber?
I believe the fitting in the back was for vacuum for the brake booster.
These carbs usually have a port for the pcv, a vacuum advance port plus a full time vacuum port.
The port in the back is usually plugged off and is for a brake booster.
Sorry I gave you the wrong info, I should have reread your post and looked at the picture closer, rear of the carb! 340 challconvert is correct if no booster just plug it, although usually there was a threaded hole near the back of the intake on the drivers side that the booster connected to.