I'm with Tech 100% on this one - you can not change(reduce) the cfm without changing the physical size of the carburetor body or adding a restriction plate (like sanctioned race bodies require - different story altogether).

That said - If you want a bit of balance between performance and economy, go with a 600cfm carb - like an Edelbrock 1406. Electric choke and very solid design. You may find it a bit rich out of the box and need to re-jet, but try it in the stock configuration first and see if you're satisfied with the performance. I've used a lot of the 1406 carbs very successfully.

Summit SUM-210216 is a 750cfm Quadrajet (rebuild) which is also an excellent carb and was GM's choice for a lot of years on everything from 305 - 454. Great response off idle and there's nothing like those big secondaries when they come to life for a bit of tire chirp in second gear and a wonderful deep growl that lets folks around you know you've got more than a timid mouse in there! This is probably my all time favorite carb but I caution people that it is way too much fun (and perhaps addictive) to kick the gas pedal and your economy goes right out the tail pipe!

Regards,
Glenn