I had a 170, not stock, in a 64 Barracuda 4 speed that would get high 20's all day long on the highway. Probably hit 30 mpg a couple times. And I can do math.
As you said "not stock". Mine is not stock either. I stated that I did not believe any stock slant six ever got "real world" over 30 mpg, and I stand by that statement.
PS: A 170 was not available in the barracuda from the factory. 225 was the "base" engine for the Barracuda, and 170 was base engine for the Valiant.
PPS: I had a 170 in my 66 Cuda, but I put it in. Removed a not running 360. Car was born with a 273 2bbl.