First, Welcome to FABO!! Second, there is no "G"(short-deck) Slanty block in Al-U-minimum that I'm aware of, so no Al 170's unless You de-stroke an Al 225. Third, the 170
"G" block is Approx. 1&5/8" shorter than the "RG"(Raised G) block the 198 & 225 are based on, so it is a good chunk lighter & still iron. Fourth, under-sizing an under-power
engine for a task will cause it to downshift much more often, & spend more time in the carb's power enrichment circuit....not more efficient.....................
The upshot is if the smaller engine has a high enough output efficiency, and the gears are picked correctly, it will return better MPG. If not, it will disappoint You..........
My opinion is to skip the Al-U-minimum block unless You just want the novelty of it, too many finicky issues possible, and You can find better ways & places to lose 80lbs.