Stalls in Neutral

[Thor64]

When I shift into neutral (manual transmission) to stop, my engine almost dies - it will if I don't give it a little gas - and the air is heavy with the smell of gas. Here are a few details:

-'72 350, rebuilt ??? years ago
-brand new Rochester Quadrajet from Summit
-brand new fuel pump, filters, and lines
-gas tank is clean as a whistle
-plugs and wires appear to be in good working order
-new vacuum line between the carb and the distributor
-HEI distributor looks old, but has been working well
-timing is good
-truck was running like a champ until this problem arose

I have a sneaking suspicion that the HEI is on its last leg, but my last two classics had points, so I'm not as familiar with this system as I need to be.

Any suggestions on what to check?

Thanks in advance!