Hi mauriciodiaz, that link didn't seem to work correctly, could you try copying and pasting it again? I guess I'm having a bit of a hard time visualizing where you've got some corrosion. Depending on where the deterioration is you may not need to remove any fiberglass, but I'd need to see some pictures or a diagram to say for sure.
Sounds like you've got a plan lined up for swapping in a 454. I won't fight you on that, I love big blocks.
I would recommend that you spend a bit of time doing reasearch on the swap though, it would stink to get halfway through only to find that a bunch of your parts won't work together. $35 for a block sounds dirt cheap, almost too cheap. Before buying anything I'd try to figure out why the vehicle ended up in the junkyard (e.g. did the engine blow into bits, or was it in an accident). It may save you from spending money on a block that's toast. Perhaps the owners of the yard can shed some insight or provide some assurance that the block will be usable.
I'm sure you already know this, but be sure to have a machine shop magnaflux the block (to check for cracks and imperfections) before doing any
machine work. Be prepared to spend a fair bit for machining work too. Depending on the condition of the block you may be looking at acid dipping (to clean the block - a definite) boring the cylinders (a definite), decking the block (a strong possibility), and align honing for the crank (a possibility). Depending on your plans, and the condition of the core, you're $35 block will likely cost you a few hundred.
Good luck with it, and let us know how it all works out.