Fuel Injection in theory is more efficient - but even so large performance engines still use loads of fuel. Ford found out wit hthe recent GT that between 4 - 8 mpg even with fuel injection was the average MPG
There wasn't much fuel injection going on in pre-80's cars. Usually mechanical fuel injection which didn't offer the same fuel economy/robustness as conventional EFI. At best, the electronic carburetor was all that was to be had on most cars, and it's usually regarded as worse that a regular old mechanical carb. You have to at least step into the mid 80s to get into any form of EFI, unless you are asking about retrofitting EFI onto your Z28 camaro, to which I would highly recommend against, if only because I doubt you are mechanically skilled enough at this point in you life to perform the work professionally enough to make it a reliable, tasteful, retrofit.
There are many that are custom tunable as well, point is, there are many bolt on options for you to choose from, and actually they are pretty decent with today's technology.
Oh, by the way, that Holley 750, is it one of the old "double pumpers"? The 750 and 850 double pumps were for performance, but they lend about the worst fuel economy of any carb ever made. Literally. You would do better if you changed over to an Edelbrock or even a Carter AFB with Vacuum secondaries. Even if your Holley is not a double pump, it is still a horrible carb for gas mileage. You will lose a little initial "bang" with the vacuum secondaries, but you will get a lot better fuel mileage if you can keep your foot out it.
In any event, you don't need a 750 CFM carb on small block, unless you are running heavily modified heads, high compression pistons, and a very, very healthy cam that has a lot of overlap, that cranks on the horsepower at 55 hundred to 7 grand, which would be miserable to drive on the street anyway. And no matter what you did, your gas mileage would be terrible. So...just dumping the Holley for a more practical carb would help a lot. I am guessing this is a small block anyway. Let me know if it is not!