Probably because all of my 327's where actually displacing 331 with 11:1 pistons and solid cams, my 350's where all 355's and they made well over 400 horsepower even back in the old days of Sunoco 260 and American White 104 Octane gas. My big blocks all had 12.5 pop-up open chamber domes and huge cams with some of the wildest carbs Holley ever sold. I had a Phase III Baldwin-Motion Camaro RS-SS with a full deluxe interior and 4.88 rear gears with a Muncie four speed. It got 1.2 miles per gallon, powered by an L-88 427 with a tunnel ram and two fours but it was a fun car to drive.
My 454 Suburban gets 6 miles to the gallon whether I am towing a 7,000 pound trailer or empty; probably due to the 4.11 gears front and rear in a three quarter ton four wheel drive truck. (It dropped down to 4 mpg when I replaced the 454 with a 502). My '89 Caprice has a 513 horsepower 406 small block with 3.73 rear gears and a 700R4. It got 11 mpg if I kept my foot out of it. My 582 big block 1985 Impala can not get more than 3.2 mpg even with a 4L80e overdrive, but it has 4.56 rear gears. Worse than gas mileage is the number of tires it burns through in a month.
My new pick-up truck has the 5.3 LS-2 motor with a six speed overdrive tranny, 4.11 posi and gets 20.0 mpg last time I checked the gauge.
So from my perspective he is getting great mileage.