Parts book says 3/8"-16 x 1-1/4", which jives with my memory from replacing them a couple years back. Note: I believe they are grade 5 or 8; I used grade 8 bolts to replace them. DO NOT USE LONGER BOLTS OR YOU CAN PENETRATE THE GAS TANK. I also used a 3/8"-16 tap to clean up the threads in the hard points in the floor before I put in the new bolts. Again, do not go deep enough with the tap to contact the fuel tank(s). I had a problem where some of the bolts had threads worn enough that they would not pull up tight. New bolts solved that.
Note: you did not mention the model year (I don't remember from the other thread we are chatting in). This info is from the 1973-1978 parts book.