Patrick is righ on about what is going on. Put some stiffer springs in the dizzy to keep it from advancing till it's over 1000 rpm. There is no reason you can't have 15+ inches vacume at idle and 12 or more in gear even with the stock low stall converter. It will drop more than a stock lo-po cam but should not be nearly as bad as it is.

PS get a vacume gauge you don't have to adjust for. They are real cheap at most parts stores.