'To be a bit more specific. I want to create a plenum and "drop the floor" to effectively RAISE (vertically) the flow.'
Buy the Racer Walsh 2 inch thick adapter, that does that and you don't screw with messing up the 1-4 entries, which is what you will be doing by cutting into them at the top to remove the turn. It is right at the top and any opening up will do that. Any more work to finish at the center of the X in the middle will only make it even worse as you get closer to flat. You need that ROUND curve to make the turn. Chop parts of it up and turbulence all over the place right at runner tops, a particularly bad area to do that. The commonly used one inch one is way too low in my opinion and why they cut deeper into the manifold trying to relieve that issue. They lose at the 1-4 when doing that, but commercially they make money. By selling a cheaper to make adapter at almost same price as the thicker one, and the look pretty intake port work at $500 to draw in the suckers. BTDT.
Of course the dance again around the hood clearance issue......... ........
With the carb right at the commercially available adapters' location you will ALWAYS have some sort of turbulence issue, there is no one inch straight below the throttle plates to allow mix to get straight to then turn lower to go its' 4 separate ways. The throttle blade angle is going to mess up everything in there until it is straight vertical. On a Weber mounted in conventional location fully one half of wide open mixture is shielded from the outer lying areas by the wide open throttle plates. Why the two inch spacer, to get vertically up some to resolve some of that. You WANT some void in there to allow for good cross mixing at bastard butterfly angles as well as to allow for some splay out further room to reach areas the carb bores are not directly over. Two inch looks right to my eye without going overboard in excessive 'tunnel ram' fashion. I wouldn't hesitate to put that on an ATX car. Maybe advance the cam a bit if uncomfortable about it.
Tunnel rams can easily be made to work at low rpm, they are in effect on almost every EFI car on the planet now. What's the difference? Fuel distribution, which can be fixed in other ways. I built tunnel ram cars with carbs that would pull stumps out at 2000 rpm. You make the plenum area and runners smaller is all.
To Dick, yeah, I know about F-104 leading edges, and was thinking some SA like you would probably bring something like that up............ LOL..........O r course we all know what the mission of the F-104 turned out to inevitably be once everybody got over all the gloss and glitter right? Certainly no stars shot down there for sure.