Some more stuff here.
There's a lot of other Ford forums (Bronco, F150) out there where guys have accomplished this same electric-fan conversion task pretty well. I've been researching it for my van's 5.8L, but the basics of what's accomplished ought to be pretty similar to your 5.4.
Some takeaways that those guys seem to keep mentioning on those forums:
1) Adequate CFM
--- for the 5.8, it seems like guys advocate finding something that runs at least in the 4000-5000 cfm range
. There's quite a bit of mention of the Lincoln Mark VIII electric fan (junkyard find), which is mentioned as being 4300 cfm unit.
On the flex-a-lite pages, you'll see that any units they offer that are rated for over 4000 cfm are starting to get into dual-fan units.
2) Well thought-out Fan Control
--- most of the bolt-in systems utilize an automatic thermostat "probe" that gets attached to the side of the radiator, and thus creates its own automatic fan control cycling, and guys who piece together their own systems have to wire in something similar --- beyond that, a lot of guys feel its important to have a secondary wiring circuit with an "override switch" under the dash, such that they can temporarily kill the electric fan's operation for water crossings.
3) Adequate alternator output --
These fans don't create their cooling "for free," they still do put a load on the engine when the fan operates (increased demand from the alternator = more horsepower required to spin the alternator.) Our vans have pretty high-amperage alternators to begin with, but it still needs to be analyzed as to whether the OEM alternator is up to the amp-draw requirements for the additional load of the high-CFM fan assembly.