Engine swap 170 to 225

Feather Duster Dan said
My slant six goal is to find one that drinks the least amount of fuel at low idle.

I'm sure that's the first time I've ever seen that as a goal. How come? How are you using your car that it spends most of its time at a low idle?

(Also, if your present carburetor works well, it will keep working fine on a 225).