Fist generation a-body, pre 1967, are different from 2nd gen'. All 2nd gen floor pans got a little longer and a little wider. More spacious was selling point of 2nd gen' a-body.
The difference in the bucket seat tracks suggest a good difference in pan depths also.
There is a thread here somewhere about homemade filler strip about 6 inches wide between the aftermarkets front and rear floor pan sections. I forget what car that was.