Really like the idea off a healthy for bolt main sbc. People are telling me I'm retared but i see a ton of race boats run them. Never see one in a drag boat but in a lot of other forms of racing. I want a motor that can handle a 10 minute hot lap id be willing to sacrafice some power for a motor that will hold together. I got this boat to pretend I'm a jet sprint racer not to do the ocasional 1000ft drag race and spend therest of the day on shore.
I wouldn't say your retarded... or even remotely so... there is nothing wrong with wanting to build something to handle bitchen, rev to the moon, and get after it lap after lap... I kinda dig it.
But... If you're wanting a sbc to buzzzzzzzzzzz lap after lap like johnnynascar... then you're gonna need to spend some loot on a stout bottom end, some really good valvetrain parts, and even of you're gonna pit it all together yourself... properly machined/balanced/and heads setup just right... any corners cut in these areas and it just won't last. Built small blocks do like to rev... but in a jet boat and in an endurance sort of application, there is just no "cheap" way to go about it... it either is going to be right and work great... or not and won't. What is a bad-ass build for a hotrod street dragnasty car typically ain't gonna cut the mustard in an endurance jet... apples and oranges.
Seriously though, I hate to be the negative guy... I know you said you wanted to find something used and cheap... and I'm sure someone will chime in and say how their buddy's third cousin's neighbor took a 305 out of a station wagon, slapped a 850 on it and the biggest cam ever made in it and put it in his circle race jet and won every event in every class and set all the records that have ever been recorded...

but the simple truth is... if you want a stout sb, that can run hot lap after hot lap and make it through a season or three... you're gonna have some coin tied up in it.
The ls route is a good way to achieve several things at once... but even then... endurance oriented application, rods and pistons need to be upgraded... as well as cam and valvetrain... but to be honest... I think in the grand scheme of things... even building it up nice, you'd be cost and benefit ahead of a typical iron sbc... but that's just my opinion.
No matter which way you decide to go... after doing this for a few years and gaining a little bit of knowledge along the way... I found the cheapest way to go is doing it right the first time... and it hurts a lot less than having to go through the effort and expense of doing it twice, three times... etc...