I don't have access to the boat at the moment. However, I do have photos I took when I was measuring the boat for my plans. You might be able to get a "best guess" from these... It extends from just forward of the transom, perhaps 8-inches, to forward of the steering wheel board, again, about 8-inches, on either side of the steering board. The round holes were NOT original to the floorboard. You can see the loop bolts that he used to tie down his marathon tanks for 100 mile open water races.
Having driven a twin to this boat, the foot holds would not have been used during normal running, rather they were used when turning or when planing off. During normal running, you would have your feet against the transom. At least I did - I am 6-feet, 4-inches tall. During turn maneuvers, you would throw your weight over the throttle, hooking your right foot into the toe hold in the floor to keep yourself from bouncing out of the boat as it rolled up on the chine. Even without a fin, the boat would turn on a dime and give you nine cents change.

Or would that be turn on €0.10 coin and give you back 9 €0.01 coins?