The EFI solutions from the major brands would be $3k - $5k with quite a bit of fab work. A marine intake swap could be done anywhere from $1k - $1500 depending on whether a used or new intake is used. The cost variance also depends if you upgrade the injectors for that intake. They come with 24# marine injectors (at 43.5psi) which means they flow around 32#'s at the 58psi that the L31's normally run. Of course, no additional cost if you use those, but add more if you decide upon larger injectors.
Honestly, if you have not done anything major to your existing block, a LSx swap would be to your benefit. Especially, if you are a wiring guru or at least extrmely knowledgeable than one of the hardest parts is covered. You should look for a '99 - '02 block because they still used cable TB's which makes it a bit easier. Shoot for a 6.0 if at all possible.
I've invested the ProCharger, DT headers, exhaust, etc...into my 5.7 already so I'm not about to swap out another block as of yet. That's my reasoning but I know I will get flamed by others telling me my 350 block is a POS compared to the newer technology.
