talk to INFO,Info (http://marineassault.net/) I spoke with him about big old EFI motors, He can make them anyway you want. (He's got good innerds too ) If my boat didn't already come with a motor I would have used him.
Also talk to Gary Taylor, he is on the boards too.GT (http://www.tpboats.com)
If you don't already have a motor and are willing to use someone else, don't bother with Merc.