Sorry for the confusion. When I wrote "extra cab" I should have written additional cab. The "extra cab" the seller has is an additional standard cab. So, I would be replacing the rusty standard cab with a different standard cab that is included in the sale.
I have yet to see the truck and additional cab and don't know if they are from the same year. I do know the running truck is a 1973 auto and the additional cab came from a manual truck, year unknown. That is a plus for me because I would prefer a manual over an auto.