Buy the floor. I just replaced most of the floor of a 55 Ford F100. (Yeah I know, please forgive me. But it was for a person that ) He bought the floor pan and I used pieces of it that I need in order to have a solid floor.
After I got it installed and painted, it looked like it came from the factory.
It was a whole lot easier than trying to fabricate a floor out of sheet metal.
Spend the money for the floor. You'll be glad you did.

Frank