The best answer will require knowing the timing vs. rpm. For example, you set timing at 5* at 750 rpm What was timing at 900 or 1000 rpm, 1400 rpm etc.?
My guess is the throttle was too far open,, so when the vacuum advance was reconnected, it added timing and the idle rpm floated up.
(Just as Chrysler explained it.)
Further guessing the engine is not stock.
I would reduce the throttle stop position (slow idle) and increase the initial a bit more so the engine again is idling at 750 or or slower if it will do so.
Plot the mechanical timing vs rpm out on the chart. (no vacuum advance connected, plug the port)
As long as the timing is between green lines and blue lines it should be fine.