Robert, like David I have a 68. I wanted to keep the crossover for the correct appearance but did not want that extra volume attached to the manifold that might disturb the air flow. So I removed the gaskets at both ends and replaced them with aluminum plates cut to shape and sealed. Because the crossover was then closed, I drilled a breather hole in it to allow pressure to equalize. I have been happy with the results.
Another note. when you remove the butterflies, the spindle is free to move back and forth. I had a vacuum leak develop that took quite some time too identify but was easy to seal.