In the end, I think you would be much happier by just doing the 99-up PI head/intake swap. You'll make alot more power than a stockish 5.0L motor, and tons more than your current engine.
The 4.6's aftermarket is starting to really come alive, so theres plenty of parts to choose from now. Not to mention the fact that that motor really loves boost
But if you just insist on putting a pushrod motor into it, and want to go through all the trouble, then why not a 351? Not really anymore work than putting a 302 in it.
Just a thought, good luck
-drew