I have an '05 FX4 and a Reese 16K slider and I don't think the ride height is different than your '06. I don't think the bed is extremely high, although I have the hitch set to its lowest position. I could probably go up one hole on the pin to get my FW to ride more level (its slightly nose high). I don't think 11'6" is very high, and I also think you could probably have the FW raised to get it level, if necessary. Almost all interstate bridges are at least 13'. You also want to make sure you have 5-6" of bed rail to bottom of FW. A dealer told me, and I believe him (this time), that it's too difficult to determine your hitch requirements without just hooking up, so that's what I recommend you do.
