Kevin is right.
I just wanted to mention that 51 doors were only made one year and are rare.
They have the 51-54 style vent windows with 47-51 door latches (pull down handles).
The 51 body needs 47-51 door latches to work without some serious modification to the body.
Identify what year doors you have.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."