MOst of the 5.0's are flex fuel engines.
Does yours have the "FLEX FUEL" label on the tailgate?
If so you can just fill up with 100% E85 & try it.
If not I would suggest a mix 50% E85 and 50% Gasoline
Since all vehicles sold today have to be compliant with Ethanol mixes of 10-15% the fuel system parts should not have any issues with E85.
However if your vehicle does not have the flex fuel option, do not use over a 50%-50% mix of E85.
Running a ratio higher than that will not harm your engine but it may set a fault code for a lean mixture due to high fuel trim numbers
Since I am an automotive engineer, I do not care if my vehicles set a fault code when trying mixes higher than 50% E85. I just hook up my laptop with the OBDII adapter, clear the codes & go on my merry way.
At any rate your engine will probably perform better with a higher octane fuel.

FYI All:
The August Issue of Motor Trend has a test of the 2014 Chevy Silverado
The 5.3L FLEX Fuel Engine in this truck is rated at 355hp & 383lbft of torque on gasoline. It is rated at 380hp & 416lbft of torque on E85
Check it out on page 122 of the August Issue of Motor trend.
I am pretty sure the result would be the same for the 5.0L DOHC Ford engine in your truck. I am sure it increases the torque in my ECO Boost engine at least 20%. Interesting??