I wouldn't let the availability of an aftermarket harness decide your motor for you, JMHO. If you want to do the 351 EFI then do it. The stock harness is usually free with the motor, unlike the Painless harnesses that are only "painless" after you pay for them. $$ $$
I've done two EFI swaps (Chevy, not Ford) and both times, with two different EFI's, I've used the stock harness. You buy the factory manual and just start deleting circuits you dont need one at a time. It's not that hard and you will get an education on your motor that is worth many times the cost of an aftermarket harness.
The 351 is a bad MoFoCo mill and EFI is the way to go. Hey, everybody's got a 5.0, right? Go for the displacement, there IS no replacement.