I took some pics of my 1972 C30 which has 44,000 miles on it which has factory bucket seats in it. Like this truck and all the trucks that we have seen bucket seats in, none of them used the bolt in the support that runs underneath the floor.
The front bucket mounting just used a square nut welded to the floor. No additional floor support brackets was used except for what was used for the bench seat. I will take some photos of the front mounting and add them later
As for the offset on the drives side front holes I found it to be 1 1/4" to the left. See photo.
