I'd have put the red backboard on top and the blue one immediately below it. That's how they are designed to be used. That way, there would be no need for the white mushroom boards at all. With the current setup, there's no way for the KSU tails to reach the blocks on the red board without crossing the plane of the jumpers, a no-no.
Of course, there may not be enough room for you to have done it that way. With that being said, it still looks mighty fine. There's nothing more professional than an installation using factory backboards. I sure wish everyone did.
I would probably have mounted the Melco intercom perpendicular to the rack instead of parallel, but only to conserve space. There's nothing wrong with it being the way that it is.
Your 66E3 blocks won't be a problem, as I have answered your PM already. As for the yellow backboard, you may be better off with a blue one and a can of spray paint. Yellow backboards were rare even when they were fully in production.
