How can I tell how much power a shortblock can handle?

thats a loaded question and a half. no manufactorer is going to tell you an exact number. they can't, alot is the tune the other parts. with that being said. a stock block can usually handle up to 500fwhp. dart blocks, the sky is pretty much the limit for people on this site. as far as your internals, can't say for certain, but i still have a cast crank and have 450rwhp now with more on the way and not to worried. look at eades5.0, he's had crazy power numbers over the years with stock bottom end.

word to the wise, the tune is more critical than the parts inside the block.