I have heard nothing but good things from Guessford Machine
http://www.gessford.com/cobraparts/fblocks.htm . They sell Shelby Blocks. Plus they are able to deal with Turk!
If you want to go directly to the horses mouth, here it is
http://www.carrollshelby.com/engines.htm
I am not sure what the price is, but I do know the block is one bulletproof piece. It looks like a Top Fuel block with all the webbing. It's almost a shame to only build it with 500 or 600hp!
Kris