a 4.2 liter 6 is around 258 cubic inches.... but I thought that they were either 4.1 or 4.3 liters. 4.1 is the Buick v6, based on the same design as the 3.8 v6 but with 4bbl carb, maybe fuel injection depending on year 4.3 is a chevy design-all of them i know of were EFI, but they are essentially a 350 chevy with 2 cylinders cut off. The 307 is pretty good on gas. mine is not as good because 1 its a wagon and 2 it is not totally stock (dejunked and not in completely perfect tune), but they have been good for 28-30 mpg in some vehicles.
My service station said they would not do the annual inspection on carbureted cars anymore, so I am okay on that.