Honestly...how good was the 318...mpg / durability / hp

1976 I had a 340 4 speed Duster and spun #1 rod bearing.
I was leaving for Tech school (Denver Co) in a week and had to fix the car fast.
Got a 318 from the wrecking yard, slapped a Thermoquad intake and carb on it.
Pulled the 3.91's for a set of 2.94. Bare bones Duster with headers the car would pull down 20 mpg on the 1600 mile trip.
Drove it, raced it, sold it to my neighbor who used it for years,gave it to his little brother who also drove it daily till it lost it's life against a telephone pole. The car was sold to a guy who pulled the drivetrain and who knows,it may still be going.
My 63 Belvedere has a 1970 318 in it now. The 318 was a underdog but tough as nails.