I ran a holley 750DP on a mild 340 (stock 340 cam) w/ zero problems. It had great throttle response and I got 17mpg hwy... I would think a 650 might be too small for your application. The 750 vacuum is ok but it will require tuning as does any holley. If it were my car I'd probably get a used 750 DP, rebuild it and start w/ 72/80's a 6.5PV and 28 squirters...
FWIW, My 72 swinger had an almost stock 340, headers, J-heads and a holley 750 DP. It ran 13.80's on street radials and 3.55 rear.