If you could ever achieve 100% volumetric efficiency (good luck on that, none of us live in Utopia), a 305 cubic inch motor running at 7,000 RPM would require a carb with 617 cubic feet per minute flow ratings.
If you can manage to achieve 80% volumetric efficiency, that same engine combination would require 495 CFM.
Do you plan on screwing that motor up to 7,000? Ever? What are the chances, somewhere between no way and zero?
So let's assume a more reasonable maximum RPM number of 5,000 (keeping in mind that cast crank). At 100% VE, you would need 442 CFM. At 80% VE, that drops to 354 CFM.
Off idle, you need an accelerator pump to crutch the near-stall of air flow through the carb. At low RPM, you need the carb to be as close to the Intake valve as possible, to maximize the vacuum signal at the bottom of the boosters. And you want plenum and runner volumes to err on the small side of things, to maintain port velocity (read: throttle response).
Walk outside, remove the nozzle from your garden hose and turn the tap on. Have you got a pretty good stream of water flowing? Now, using your thumb, cover about 1/2 of the end of the hose. What happened? Spraying water out into the street, are you? Bigger is not better, unless we're talking wide-open throttle.
Need another example? (And this one will help you understand what that big plenum is going to do to you.) You are going to need a helper and a drinking straw. Have your helper blow as much air through the straw as possible, whilst you hold your hand 3-4 inches from the straw. Feeling a pretty good stream of air? Now, repeat the experiment, whilst you hold your hand 3-4 feet from the end of the straw. Can you feel anything on your hand? This last time, you hold your hand up on one side of a room, whilst your helper does the huff and puff routine at the doorway of the room. How much air do you feel moving on your hand?
When an intake valve opens and the piston starts moving downward, it creates a vacuum at the top of the cylinder. That vacuum signal will travel through the intake runner (if bigger was better, why isn't that intake runner 4.000" in diameter?) and then the signal arrives at the plenum. As the plenum has more volume than that intake runner, the signal loses strength before it makes its way to the bottom of the carb, where we need it to do its thing. If the signal is so weak at the bottom of the carb, the carb can't accomplish what you want. Carbs have no brains, they're dumber than ordinary garden dirt. If a carb senses a pressure differential between the top opening and bottom opening, it will start to flow air and as that air flows past orifices, it picks up fuel, carrying it to the cylinder.
What this all boils down to is this - tall intakes with big plenums work well on race motors. Short intakes with small plenums work well on street motors. Does a 180° single 4bbl intake have the same sex appeal as a sheet metal tunnel ram with a pair of split Dominators? Nope, that race setup is what gets our blood flowing, isn't it?
I suspect you could take a smaller, unmodified tunnel ram, pillow up the plenum to reduce volume and weld up the runners to eliminate some cross section and really make it work better on a street application. But you're still going to want wee carbs and that means a pair of 390 CFM carbs are still going to be too big for your 305. So now, it's time to start tweaking on the accelerator pump and main jet circuits, to get the carbs to work the way you want. How much time and money are you going to invest in that process, when you can simply buy a single 4 bbl intake and a 600 CFM vacuum secondary carb for a lot less money. That vacuum secondary configuration will keep throttle response where you want it and the carb can open the secondaries as needed when you want to play. With the money you just saved (have you priced 390 Holleys lately?!?), you can keep purchasing parts that will make real power.
I reckon the question comes down to whether or not you want to look good and give up some driveability, or have a more tame appearing combination that will rip your head off when you stab it. To each his own, aye?
When people ask if a tunnel ram can be made to work, that gives me a clue they have no idea of how to make one work well. And if you don't know what you're getting into, why not stick with something you do know? You can always work yourself into unforgiving combinations when you gain some experience. Take a look at GT63's blower/EFI setup. Are you ready to sort out that kind of combination? If you are, go for it, because I'm betting you can have the best of a lot of worlds. I would love to play with that combination, myself. But I will still opt for a single 4 bbl. combination, because I want simplicity from the get-go.
