That $5000 is because Shelbys block is aluminum (and has his name on it!), Genesis' has iron ones for $3500. Considering these new redesigned blocks are superior in every way to the originals thats not a bad deal.
Blocks at Genesis427.com
Their aluminum block is only $4695.
BTW if you really want power(and it doesn't matter if the engine type is correct for the year/model) forget the FE and go with a 460! Dollar for dollar it's a no brainer.