As much as my first reaction is 351 with football rods and hogged out GT40s or maybe some D0 heads or something... probably would have to do a 302 roller with the forged pistons, hogged out GT40s, ported intake, GT cam, 1.7 rockers, FRPP shorties blah blah blah.
Not sure why you would want to though unless you are running a stockmod class or some such.
Now for a MODULAR... now we are talking. Terminator longblock, port the M112 and put a smaller pulley on it with a tune and hello 500whp.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.