About 1968 - 1970 was the period when the US makers actually were forced to start looking seriously at reduced emissions, and their need to make "bigger" cars compliant faced somewhat larger obstacles than for the little riceburners.
EGR took some of the snap out of typical engines then, but the thing that really hurt performance - for models for a few years - was the intake manifold preheat, used to get a constant inlet air temperature so that the relatively crude mixture controls available in mass production quatities could achieve a true "lean burn."
I had a 1970 Impala with 350 engine that was such a dog that I traded it off for a 1971 Chevy wagon with 400. The rated horsepower for the two engines that year was almost identical; but my thinking was that the 30% higher max torque from the 400 would help - even if the car was 600 pounds heavier.
The prediction worked as expected, and "performance" was much improved, but both cars hovered at around 10.1 mpg. Running with inlet air at 140+ degrees (F) just doesn't let you get the same efficiency (or the power) as with colder air. Fuel economy was traded off for reduced hydrocarbon emissions. EGR mixed "mostly burnt" gas back into the inlet which theoretically "finishes the burn" but also gives the appearance of richer mixture and suppresses knock, so the compression ratios could go up a little bit while still using leaner mixtures.
I learned - almost by accident - on long trip on a road where all the stations were closed and my tank was getting really low - that just flipping the lid on the air cleaner upside down, to leave a crack that let in cold air, converted my '71 wagon from a 10 mpgh "gas sucker" to a 15 mpg road runner. (Not impressive now, but a pretty spectacular difference back then.)
That was on a rather cold night (10F ~= -12C), and results might be less obvious at more normal temperatures. I did do some "controlled tests" later and confirmed the "fuel economy effect" of the reduced inlet air temp.
Since with a cold inlet you stuff more air into the cylinders, you get higher compression peak pressures, and with the lean carburetors of the era you do risk getting more knock than is good for things. Several people I know who "disconnected the EGR crap" to "make it all better" did experience a few blown gaskets and warped/cracked heads that I heard about. Of course they refused to believe that their "modifications" had anything to do with it.
The flipped lid on the air filter, and colder inlet air, on the night when I first tried it also had the "side-effect" that the engine ran so cold that the temp gage stayed pegged at the bottom. Since the thermosat never opened, the heater didn't work - at all. (This produced some really objectionable noises - from the passenger seat areas).