...the two-piece "split down the middle" frame was dumped in favor of a one-piece lower chassis with separate roll cage (much like the MPC units).
Someone who knows told me that Revell had two chassis tools for the original version funny cars; the frame was redesigned and the remaining parts were culled from the two original tools, creating one "new" chassis tool. Tires were also new. Only one tool was needed since Revell wasn't issuing as many kits, and each of those wasn't selling in the numbers Revell was moving back in the day.
I believe the first kit with the "new" chassis was the Blue Max Mustang which also had a new body (Revell didn't sponsor any Mustang bodied cars in the Seventies). I'm pretty sure all of the reissued kits after that used the "new" chassis.
--Mark