Back in the day NHRA said nothing about what you could or couldn't do inside the manifold.
All they cared about was how close the carburetor/s were to the factory location.

I saw plenty of manifolds go through technical teardown at points meets and national events that had been modified inside - they all passed tech.

Oh, they would. Butch Elkins of Diamond Racing Engines would modify the factory Cross Rams by cutting the center section off the top and increasing the interconnection area. Worked well, and was legal. Once in a while you will run across one, you can tell by the raised pad over the interconnection area in the center, where the stock was cut out, and a new panel was installed.

The Vanke intake was a stock inline, all opened up inside. Dick Landy did that, too. It took some off the bottom, but gave a lot back on the top. It was not far off the Rat Roaster on the Street Hemi cars.

It was so, so different. Better, too.