Experience purely from reading...
That jet can only pump the water it can get to and then it needs to get rid of it properly. Hull alignment, loader grates, stuffer's, droop snoots, place diverter's, FT nozzles, ride plates, set back pumps, etc. are each huge in their own right.
The stock marine Olds typicaly have higher HP than the stock BBF's and the stock BBC's. The Olds are notorious for oiling problems, especially in marine applications. The oil doesn't return to the pan fast enough. BBC's are cheaper and more plentifull to build than the Olds or the BBF's.
Prepare to spend a LOT of cash if you want to be one of the fastest out there.