I would go with a DEAN GELLNER 750 EFI like in last months powerboat mag. DEAN MAKES A NICE reliable motor theres a few race teams that have gone more than a season on his motors. I having a 572 blower being done now, and will have him up grade my 670hp motor next winter. I have seen troubles with the 496 and 525efi merc . a buddy had one replaced after the 1st season and now one of them does not like to start all the time so merc is not perfect know one is. but they cost more than others. ps the headers on the merc stuff is made cheaper by cmi for merc but costs twice as much as cmi etops or stainless marine gen 3s which are much better I think.