Are you building a drag car? Or a street car? Will you current suspension handle 500-600 hp? If no, is there room in the budget for another $1k-2k for suspension upgrades? If you're wanting a fast street car it doesn't take 600 hp in a mustang, it's not a Honda,. You'll start making power around 3k rpm vs the imports 6k. If you were to make like 350-400 at the wheels you'd be able to hold your own with most on the street as long as you're not running a 2.73 gear.
If this is a street car my vote is investing the money in the 306 and shooting for a lower target hp goal and spending the money on the other areas that will need attention after adding an additional 100-200 hp, ie rear suspension. Oh and no need for forged internals, if you end up making "block splitting power" the block will split before the rotating assembly.
Mod motors- $2k cam upgrade...
Don't Sext and Drive.