Got an original J2 set on my 57 Olds (three two barrel Rochester 2G's). No tuning problems, only have to set the center carb, the others will join in when you want them.

My engine is the bb Olds 371, and my mpg ranges 6 - 8. But you likely will have a smaller engine, lighter car and maybe a overdrive tranny, so your mpg might be much better. But most 2 barrels will be larger and thirstier than a small 4 barrel with small primaries.

And yes the 3 x 2 look is a winner, but at a price.