There are no comparable classes to these being run anywhere that I know of, so there is no way to compare records.
The cars started out as stockers (stock cubic inches, with blueprinting and modifying allowed) and were based on factory weight divided by factory cubic inches, and then further by carburetion type and also by transmission, as I recall. There was a two barrel division, a four barrel division, and finally a multiple carb / factory fuel injection / factory supercharged ~all lumped together~ division. Internal mods were pretty much unlimited.
I used to race a 67 Bel Air station wagon, 283+060, Forged Trues, Crane solid roller, Edelbrock STR-10 cross ram with two 650 cfm Holley two barrels, with an A1 powerglide, 5500 rpm converter, 5.57 gears and 9 inch slicks. All that and all it ran was was low 12's. Really just a bunch of bracket racers, but it kept us off the streets and busy on Sundays.
This is exactly what the IHRA formula class was. Cars were classed on weight per cubic inch and induction system. I think the heads could be worked maybe an inch into the intake ports and below the valve job into the bowls. Unlimited head work if you wanted to move to a different class. The heads had to be factory available, the 292 turbo head being the best choice for a sbc at that time.
I cannot see enough to read the article but the 2 barrel on the Chevelle was most likely the 6425. It was rated 650 cfm. Bear in mind that a 2 barrel cfm rating and a 4 barrel rating is at different rating of Hg. Four barrel carbs are rated at 1.5 inches of Hg while 2 barrels are rated at 3 inches Hg.
The 6425 was quite unique. I had a 50 cc pump, no boosters in the venturi. Fuel was metered by small holes in the smallest part of the venture which would also be the point of lowest pressure. Jetting was usually best at 82-85 range depending on efficiency of the header system, baro pressure, and camshaft.
The 6425 was about .12-.15 slower in the quarter on a 10.50 car than a pretty good 4779 double pumper. Holley did not recommend their use under 4000 rpm due to the lower air speeds and resulting poorer fuel metering signals.
This brings back a lot of things I didn't realize I still remembered from racing in the 70's and 80's. It sure would be nice to be able to go back.