I ran EMI manifolds for several years, first on a small block then a big block. I had them make up some nice s/s risers custom fit to my boat. They did great work on the risers and were very fairly priced, but I always had trouble getting the manifold/riser gaskets to seal. The water passages in the manifold were so troublesome that I finally had a friend weld them shut. After that I machined them flat in a mill and they never leaked again. Unless they've changed their design, I couldn't recommend buying those manifolds. If the IMCO does not pass water through the flange then I'd go that direction instead. The IMCOs are also available in a version with larger individual runners for big motors.
-brian