Cast crank that you have will hold that power fine if you keep the RPMs at or below 6500. $160 for Boss rods and $500 for the budget forged MMR pistons. So $660 plus any machine work on your cast crank. You can often find Cobra cranks for cheap though if you look, any Cobra crank from 96-04 will be forged as well as cranks out of 5 speed Machs. I was able to pick up a Teksid out of a 98 Cobra with the forged crank still in it for $250 locally if that gives you any idea on price.
Or if you want some extra insurance, don't get Boss rods and get the $299 MMR budget I beam rods as well which is probably your better bet.
Just remember, budget and 600hp do not go together... make sure your donor car has the suspension/driveline and fuel system to support that power as well as all the safety equipment.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.