I can't remember where I have seen them either. Do Edelbrock not make one to suit?
Another option that comes to my mind is maybe using a six-pack bracket?I think the six-pack manifold is about the same height as the RPM but I am not sure if it is the bracket that is taller or the mounting position.
http://www.lokar.com/ make a very nice system that would work but is a bit expensive.
But can you not just use spacers under the existing bracket and longer bolts?