With only a 4" hole in the firewall and the engine completely sealed at the front breastplate, I would be concerned the engine might not get enough air at higher speeds. The fan will require 1,000-1500 cfm by itself at highway speeds (3,000-3500rpm) and with another 100-3 or 400cfm drawn by the carburetors (depending on throttle position) I don't think a 4" hole (which is just under 12 sq.") is going to be enough.
Not only do you need enough air to feed the engine, you need air flowing through the engine compartment (and out the grille) to remove the heat radiating off of the engine sheetmetal and components, so the air feeding the fan and carbs is at or near ambient temp and not already pre-heated. If you have heat control issues you'll know where to start.
I completely agree the engine bay needs a lot of air.
Al and I have a gentlemanly disagreement regarding that the design of the car is meant to move air through the engine bay (in... somewhere, and out the rear grill).
The figures quoted above regarding the engine's need for air indicate to me that the idea has to be to supply the engine bay with air so the engine can use it. The engine itself has a voracious appetite for air. It is being drawn into the engine through the air-cleaners, or across the heads and out the bottom for cooling. I'd say Al's figures are conservative: a big Type 1 at speed "eats" 2000-4000 cfm of air.
Air flows into the engine compartment through whatever is available, either down the carbs and out the exhaust, or in the fan and out under the car.
The problem is that almost every available space is a low pressure field- behind the car (under the car, in front of the firewall, etc.)-- all of it is a low pressure zone. The "suck" of carbs and fan have to be strong enough to overcome the low pressure in the places the engine compartment is getting it's air. That "suck" is constant (rather, consistently variable, depending on RPM and load), but the pressure zones vary with vehicle attitude, speed, accessories, etc., and they vary wildly (pressure zones decreasing/increasing to the square or cube of vehicle speed, etc.). I like to drive fast- this creates really, really weird pressure zones.
Terry Nuckels reports that his engine runs 5- 10* cooler with a suitcase on the rack than otherwise. I know from lots of high-temp, long distance driving that just popping the engine lid is often the difference between running cool enough or not. I'd contend that the shape of the car had no wind-tunnel or aerodynamic engineering done, and that the deck-lid louvers were put there because any path for air to get in is better than no path for air to get in. I have no idea how strong the low pressure field is above the deck-lid, but I'd imagine that anything that disrupts (weakens) it makes the engine run cooler.
Thinking about it more, Terry's suitcase becomes a spoiler, taking the extreme low pressure field from above the deck-lid, and moving it further back behind the luggage. My popping the deck-lid accomplishes the same thing, to a lesser extent. If I could pop the front end up a couple of inches to create a "scoop" like the "Cal-look" VW guys, I could probably forget thinking about the entire thing. Pity that all solutions are not "racy" enough to look cool. I'm sure my wind-block screen makes the situation an order of magnitude worse.
What I've found in trying a lot of stuff is this: any path for air into the engine bay is better than no path. If I really want to make this work, a "scoop" of some sort is going to be needed. More is better, but little bitty fans and ducts are a waste of time and money.
My current theory would involve a couple of "ram air" 8" ducts running from the bottom side of the car (with scoops as far outboard as possible), up and through the firewall, but it's all a guess since I don't own a wind-tunnel.
But I'm 100%, fully, totally persuaded that the only air that matters is the air that comes into the engine compartment and goes down the carbs or into the cooling fan.