Hi, carburators are not so simple to change between engines and cars. It can be done and I've done it before, but you shouldn't change the diameter significantly. Carb is all about airflow and if you put diameter too big fuel won't be atomized properly and your mpg will drop, if you put carb with diameter too small for your engine you won';t get enough air and engine will suck the fuel because of huge vacum in the intake mainfold.
So stay with the similar diameter to the original one. What you can do to improve FE, first of all you can put spacer between carb and the mainfold (required anyway if you want to put different carb anyway, because of mounting points. You'll have the advantage of better fuel atomization. If you want to have better low rpm response put straight pipe at the inlet of the carb (it can be bended to fit under the hood of course but try to stick with the diameter as close as possible to the diameter of carb's inlet. The longer the better. You'll have slightly increased pumping looses but much better low end response.
Or if you want you can switch to EFI with Megasquirt, it's a lot of fun and it's always an interesting project. But not $ efficient though
