After doing some research, I think I'm going to run the 3.73s in my current axles for a while and see how I like them. Since my trailer will be light (~6k lbs), I figure I should be ok.
If I decide to go to 37s, then I'll go with 4.10s. The front axle is stripped and almost ready for paint.
Steve,
I know you do thorough research but I think sticking with 3.73's is a mistake. The 7.3's don't mind higher rpms for extended periods. The E-series as you know is non-intercooled and is equipped with a smaller turbo. Any tuning you do to compensate for the tires, gears, additional towing loads, altitude and general driveability will be very mild and won't give you the bang for the buck that gears will.
The gears also have the benefit of reducing drivetrain stress opposed to tuning which can potentially decrease reliability/longevity.
I know you haven't expressed much in the way of programming but I think that if you continue on with the 3.73's you'll be looking for more punch, mileage, lower egt's, etc.
Tuning in my opinion is fantastic but why not install fresh 4.10's while the axles are apart and then move on with power adders.
The hill climbing and slow trail finess benefits of the lower gears would be reason enough for me to swap.
It's not a like comparassion but my old '94 IDI-T with 4.10's, 285's and an E4OD runs away from Christian's '03 7.3 with 3.73's, auto and 265's.
He is completely unloaded and I have a full truck box and over 1,000lbs of fuel onboard.
Either way I'm happy to see this coming together for you.