Air cleaners!

[Jeff Lee]

On my stocker AMX I ran the factory base (your basic round tub) with a K&N 12" with the K&N pleated lid. This is with factory Ram-Air. With or without the filter set-up it ran the same. Since I live in dusty Arizona, I left it all on. Now that I'm in SS and run a taller intake, it no longer fits. Eventually I will find something that works.
For your single 4bbl HEMI, I would look at running a MOPAR "Coyote Duster" base. It is the largest base produced (other than factory MOPAR shaker) to keep heat away and it has a velocity stack shaped entry which should aide in airflow. These were the factory set-up on cars like 383 Road Runners, Super Bee's, etc. Year-One sells a fiberglass version for around $100. Since it is a mopar item you should be able to use it. You won't be able to seal it to the '65 scoop as it was not a factory deal. Or at least that's the way I understand it.
Was there a sealed scoop on those NASCAR HEMI single 4bbl cars? I don't know, it's been so long since I've seen one of those cars...