I've got the Ecoboost, 3.73 gears and my TT is only #5000 loaded, but I can cruise up the grades in 4th gear, pulling around 2500 rpms and going about 60 mph with LOTS of power to spare.
Your 5.0 is going to want a few more revs to get into it's peak torque range, so I would suspect you'd be in 3rd gear pulling around 3500 or so rpms?
On the flats, I can cruise in 6th, but sometimes 5th is preferred if I find it's downshifting anyway. For me, 5th gear at 60 mph is around 2000 rpms.
You will probably find that to be about the same for your 5.0 on the flats.
Don't reduce your tire size... That will also reduce it's load capacity in most cases and not really net you all that much in final drive ratio..
I've got the optional 20" wheel/tire package and they are the same height as the 18" combo.. I know, because the OEM spare tire is an 18" combo.. :)
I'd tow up to about #7500 loaded weight with my F150. I know it could tow more, but I like a comfortable tow, so that's just my personal limit with this truck..
Good luck!
Mitch