David, your system is just beautiful. If I understand this is a "one off" manifold? I came across another manifold from a company in Australia called EFI Hardware. It also has a built in plenum. What are everyone's thoughts on this one?
Thanks for the comment. Indeed, the carbon x-ram is a "one off". Built for the Panoz racecar from the late 90's. They did lots of experimentation on the injector placement as this can have a significant effect on performance. The intake is, as you'd expect, amazingly light!
(And the engines are from the ex-Le Mans ADA Pantera. So a big pile of interesting history there!)
As George points out, there are simple and clean solutions to provide a common vacuum signal. Personal preference, but I think these would have a better aesthetic vs the chamber cast directly into the intake. Also lighter - if that matters. Here's the vacuum set-up I use on a Ford 341: