I guess I don't understand why an early A with a slant can't get 30 mpg. I'll know in a few years when I finish upgrading mine. With modern engine controls, it should be as efficient as a 4.0L Jeep engine. I will have to live with a non-lockup torque converter though. I have electronic ignition already and plan EFI and spark control eventually.
The 198 should give slightly better mileage if everything else is comparable. Most important for mileage is compression ratio, intake valve opening (less lift better), spark timing, gear ratio (rear end), and carburetor (smaller better).
