A couple thoughts here. First, what exactly is your budget? If you have a couple thousand dollars you can find the parts you need and take them to a good builder and have something assembled the way you want it. What is in your boat's future? Is it going to be something you want to keep for decades, or can you see yourself looking for something bigger or nicer in a few years. If you don't think you will have it forever, I would switch to a BBC because you will have a much harder time selling an Olds powered boat to anyone else. If you decide to build the Olds drawbacks are going to be that the shop has to be trustworthy and quality oriented. You will still be on the hook for final tuning and break in unless you want to pay for that stuff too. Money well spent IMHO.
Another route (the cheap ass bastard method) is start searching the old school wrecking yards and/or Hemmings for a Toronado motor. They are the only Olds motors that got the good stuff every time. Gutlesses, 442s etc. some did some didn't. Olds motors are very simple to build as you don't have all that many choices for parts on the cheap. E-brock heads are not worth the money, OEM rods and cranks are fine, pistons are cheap, but there is some exotic valve train hardware available. Then put it together over the winter months, saving undue damage to your wallet.
But let's be honest here. You are never going to beat anyone in a drag race, and you are always going to be having to keep an eye on your oil pressure. I was in your situation a while back and I was able to find an almost new motor with roller rockers, forged pistons and N crank, and big oil pan for $500 from another member here. I could still see the hatch marks on the cylinders ! Honestly if I was in your shoes I would probably scavenge up the parts and ship them off to a builder here in So Cal or AZ that has decades of experience with jet boat motors. Someone who will be willing to spend some time on the water if need be with you. Carb tuning, timing, etc are all qiute different in a boat sometimes because of the load they operate under. Talk to those guys on RealOldsPower.com, they will be able to help you if you need stuff.
And remember always that "Good ain't cheap, and cheap ain't good." The low bidder is not usually looking out for you first before themself. Proceed accordingly.
