I ran into those anti-theft screws on my St. Regis back in the early 80's.
Needle nose vise grips and some obscenities for lubricant and they spun right out. Found some proper screws at the local Ace.
With the Heat button depressed is ALL the airflow directed on the windshield?
When switched from Heat to Defrost does the airflow become greater at the windshield or remain the same?
In normal operation on the Heat setting a small amount of air is bleed off towards the windshield to prevent fogging.
Back in the early Eighties I mounted two 6x9's in the package shelf of my 1959 Savoy with NO cutting or drilling. Mounted from underneath like it was designed for it. Print this template at the following link or find your own and see how it looks...
Water flow in a big block heater hoses is the same direction regardless on the body it is installed in. The front or forward pipe is the return line from the heater circuit.
This page from the 1968 Plymouth (top pic) shows the direction of flow in the heater hose as they attach to the engine. I would not assume the hoses connect to your water valve in the same order. My previous post should help you figure out the which pipe is the inlet on the valve, and then...