The number of holes per side can vary on a dual plane intake. Some years, 4 holes, other years, the oblong holes you mention. All depends upon what the factory engineers were trying to make happen.
With the fast-moving air through the carb, with a VACUUM in the intake manifold during part-throttle operation, NO puddles anywhere. IF, during WOT operation, some puddles might happen in the plenum area, then as soon as the throttle is closed and intake manifold vacuum happens again, those puddles vaporize (which might cause a temporary over-rich situation (a few seconds at best?).
One of the things the number of holes might reference is "plenum volume", TOTAL volume below the carb itself. The 4 holes can decrease it, as the oblong holes might add to it, effectively. This can also relate to velocity of the air/fuel mixture in the area between the carb base and the intake runners, too. Some might desire more velocity up to a certain rpm, as the larger total volume can aid top end power (which ONLY happens at those elevated rpm levels). On the street, throttle response matters more than 5000rpm horsepower.
The problem with carb spacers is that they decrease hood clearance between the air cleaner and the underside of the hood (and hood insulator). Use the OEM-style carb gasket and see how things go BEFORE you start looking for anything else.
When these cars were NEW, and gasoline was "real", "hot soak percolation" was a problem then, too. NOT specific just to the current lower Reid Vapor Pressure fuels with ethanol (which might make it worse). In other words, you could well be chasing "a fix" which didn't exist then and probably doesn't exist now. I will also say that it seemed that Chrysler products seems to have more of these "harder starting when hot" issues than Ford or many GM products did, back then. GM tried some fixes with heat deflectors under the carb, but it all relates to underhood head accumulation more than anything else. Just a part of the breed, from my observations of growing up with our '66 Newport. And later with my '70 Monaco 383 4bbl.
As for your "floppy" heat riser valve, there should be a thermostatic spring on the back side of it, which is obviously missing. Better to have it "easy to move" than otherwise! The valve itself is offset on the shaft, so any gas flow over it will keep it open. With the pressure pulses, it might bang a bit, though.
Just some thoughts,
CBODY67