A MPG B23 build would be interesting! With lesser reciprocating weight, as you say, alongside some newer technology, I.E. better 4 nozzle injectors with a better ECM that gets rid of the primitive LH/Chrysler system, and I expect it wouldn't be bad at all. That, and lesser unsprung weight, and that would be one efficient brick (theoretically).
Yeah, I feel that so many ratings are just too good to be true. Unless one was willing to hack a FWD setup into a 240, I doubt they are capable of 30+ MPGs. On gas, that is. Diesel seems like it may be a good option for these cars. The Ford 16v non turbo 2.3L engines dont actually get much better fuel mileage in a similar weight Ranger than our 240s. So I'm guessing, that, yes, the EPA did something with the 2.3 ecoboost estimates.
The 6.0 LS that I am building up for my 86 IROC, is a 10.7 CR, and will likely out do both my Ranger and 242 in MPG easy. It's NA, but the LS platform is so efficient.