My first question is just how healthy is the 455? Is there a vacuum leak somewhere? How well was the exhaust routed? What year is the engine? Is the distributor advance working correctly?
As for suggestions, after making sure the fuel system is not leaking, the next step is to adjust the carb. Number one is are the correct metering rods for the application installed? Once that is done, adjust the idle mixture. Most people set the carbs too rich. The idle should be dead smooth, and on a knife edge. About 1/16th of a turn either direction should degrade the idle. There is a knack to it. Next, check the timing. If the engine is older, check for play in the timing chain. If freshly built, the cam could be off by one tooth. That's all it takes. Set up right, it should be doing about 20 -25 mpg on the highway when driven gently, but not too gently. Also, the stock rear end may be too high numerically for the torque curve. the Buick 455 is a torque monster. You need the revs down in the mid teens for good mileage. Overdrive may be the way to go.
As for the fuel injection, if the throttle body will mate to the Buick intake, it should work. You might also try the F.A.S.T. throttle body bolt on. It is self tuning, and then you can optimize from there. You might also consider the Edelbrock Performer (BUT NOT RPM) intake. It is designed for torque.
I would bet that is everything was done right and working properly, you could get the wee beastie matching a Honda Civic for mileage. Good luck.