I found this:
The reason for this difference in the 1:1 STD-vs-HO is all in the camshaft timing, lift, and duration. Cylinders 5 and 4 , and cylinders 3 and 7 fire really close together as matched pairs. When the H.O. version was cooked up, a number of customers complained of a rough idle. Even in the heyday of factory hot-rods, a lot of folks wanted smooth idle.
Ford engineers quickly worked this out, and found that swapping cylinders 5-4 with 3-7 got rid of this, thus eliminating a customer satisfaction problem.
No overall performance difference can be felt with either wiring order. No kidding.
edit: oh by the way, if you look at V-8 firing orders overall you will find that Ford's cylinders physically fire in the same order as GM and Chrysler's, Ford just numbers the cylinders differently that's all."
BTW, my cam has the 351 F.O., Dema Elgin suggested it for a "race" engine.
1966 Mustang 2+2
"Money can't buy happiness, but it can buy a race car. And I've never been sad in a race car!"