Just as Vinnie said, what is the base timing set at? With vacuum advance disconnected and plugged, base timing should be 12 degrees before top dead center. Once set, lock distributor. Reconnect vacuum advance to a manifold vacuum source, this is a source that pulls vacuum full time including idle.
The correct port to use on your carb with you standing in front of it will be on the front right side. When you connect the vacuum advance right away it will idle up, then lower your idle speed back down to around 700 RPM.
Next readjust the idle mixture. Without vacuum advance and with the base timing set low what happens is the throttle plates are opened so far the butterflies are off the idle slots so it's forced to pull fuel from the main boosters at idle, this is likely what's causing it to go almost dead under load as it's simply loading up. When you reset the timing and use the vacuum advance you'll find your ride will have a nice lopey idle and it'll have far better low end power. Tuning a hot rod has almost become a lost art as so many believe 12 degrees at base is all you need and they believe vacuum advance is a emissions device that hurts power and they disconnect it not realizing it's vital to tuning it correctly.