I don't think that you're going to get a race shortblock with a World block for $6400. I suppose it depends on the quality of parts that you want but even the really cheap parts are going to cost more than that. Also depends if you want the oiling system included, timing chain or belt drive, etc.

Lots of engine builders so it shouldn't be too hard to see what is available. Just depends on how much power you want and which heads you're going to run.