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If your adapter is relatively short (like your 1/2 inch plate of aluminum), the airflow will need to make a couple of very hard turns to go from the BBD venturis to a single circular hole.
I don't follow that at all. The one 2 inch hole is directly below the two small holes. I don't see any hard turns. And momentarily it expands into the non-descript space of the manifold plenum.
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You'd probably need a plate a couple inches tall to get a smooth transition, and that will affect plenum volume and hood clearance. And getting that adapter to have smooth transitions is going to be very tricky.
Plenty of work but not that tricky or complicated. Other than plain effort, it's harder to describe than to do. Here's what I had in mind, using 2 plates which makes a 1 inch transition.Put simply it is this. Cut carb holes in one plate and a manifold hole in the other - bolt them together and grind out the differences between them.
Put less simply, I see 2 possible routes. I would not recommend the second one. The boring can be done with a slow drill, top quality (Milwaukee?) hole saws and plenty of milling lube and patience.
1. Bore the 2 inch hole in the manifold flange.
2. Line two appropriately sized (3 x 5?) plates together and drill them for the flange studs.
3. Between the stud holes, clearly scribe a 2 inch circle matching the flange hole in one plate, but do not bore it.
4. Make 2 one inch holes in that plate, oriented in the same direction as the 2 holes in the carb. These holes must fall exactly within the 2 inch circle you scribed earlier.
5. Cut out the web between the 2 holes, leaving one oblong hole.
6. Clearly mark 2 hole positions in the other plate, oriented the same way, matching those in the bottom of the carb, but do not bore them.
7. Make 2 (carb sized) holes in that plate, but not at the marked positions. These 2 holes would be centered closer together than those in the bottom of the carb, by a specific amount: The outside measurement of the two holes together minus the 2 inches.
8. Cut out the web between the 2 holes, leaving one oblong hole.
9. Bolt the two plates together. This will leave overhanging material on both plates, some from the ends and some from the sides.
10. With a tooling burr (or a good round rasp) work from the manifold side (with the shorter oblong) to extend the opening out to the 2 inch circle you scribed and on a slope to the width of the oblong hole on the opposite face.
11. From the carb side, work to extend the hole out to the circles you scribed and sloped to the diameter of the 2 inch circle on the opposite face.
As a separate step (or steps), you can do whatever I left out.
This method should leave a completely smooth transition. A possible easier way to do this would be:
1. Bore the two plates to exactly match the carb and the manifold. This would leave larger holes.
2. Remove the web in the one and grind out any overhangs - a lot less than the previous way.
3. Fill in any gaps with plastic aluminum - a move that might end up dumping pieces of the plastic aluminum into your cylinders.