I don't think you can get the "MAX TOW" option with just the 5.0?? Once you select that option, it tells you that you need the EB engine option too..
If you get the "HD PAYLOAD" option, I'm pretty sure it comes with the "MAX TOW" too.
It's been awhile since I played with the "you build it" on the Ford web site, but that's what I recall the deal was at that time.. Might be different now??
Good luck and wish I could be shopping for a new rig!
The Max Tow package is available without the HD payload package and is available with most cab/box configurations.
The HD payload package (8200lb GVWR) is a available with both engines but requires the 8' box with the supercab or the 6.5' box with the crew cab.
Back to the OP's original question, either engine would be capable of pulling the trailer with at least the max tow package. With that said, the EB would provide a little better performance. The EB has 420 ft lbs of torque where the 5.0 has 380.