26' 2 axle box truck GVWR is less than the sum of the GAWRs? Am I overweight?

Yes. They can. I will use our operation as an example. All cdl drivers. We had FL 60’s with 250 horse Cat 3126’s and 6 speed spicers. All visually identical. Two trucks built with 33,000 gvw chassis and two built with 26,000 gvw chassis. All were registered at 26,000 to keep them under cdl requirement. We ran normally around 29,000 gross on all but sometimes 33k on the 33k chassis was common. Anytime we were over 26k gross on the 26k’s if it was over 4000 lbs they would fine us and put us oos until we could get the weight off to get legal on chassis rating. If under that usually around 2,500 or 3,000 depending on everyones attitude that day we might just get fined and no oos. If it was on the 33k trucks they would just ticket us for being over the 26k registration and let us keep on trucking down the road because the chassis was rated for the weight just not the registration.