Motor is a 1989 302 H.O. with edelbrock aluminum intake, flowtech long tube headers, and flowmaster dual exhaust. T-5 transmission. Will a double pumper just bog off the line and kill gas mileage? car is a 1982 mustang GT, almost exclusively for street use.
A 650 Holley, particularly a worthless double pumper is way too big for a tiny 302 with standard ring an pinion gears. 302 Fords also got into trouble with their large intake ports which were too big to keep the velocity up. When you paste on a "what ever" Edlebrock intake it'll bog like you were dragging 500 lbs behind you. You have no cam and cylinder-heads with ports that are too big. Forget fuel economy cause you won't have any.
It's going to bog without a lot of expensive modifications by a noted carburetor specialist. You'd be much better off with vacuum secondary 600 cfm. Holley. Intake velocity is the name of the game on the street for good throttle response.
What if I told you a 360 inch Chevrolet with dart platinum heads, any flat tappet cam, any intake and a *Holley 500 cfm two barrel would make 540 hp. @ 7,300 RPM.?
Why would you think you need a bigger carburetor on a smaller motor with 315 less horsepower? The trans-am 302 Mustangs had some success on the road courses where rpm could be hept up high but zero stock class wins in drag racing.
You do not need a double pump for street use on a stock motor, you fuel economy will suck. Go with an Endelbrock or Holley single pump or disconnect the secondary pump, you don`t need it even under mid street racing. Carb size is about right up to 5500 rpm
You don't actually need a double pumper, but a 650 is just fine. 2 barrels on the start and the 4 when you need it. Yes gas mileage will suffer with the double pumper. You do not need it unless you are running a HOT cam, or under racing conditions.