I have a '07 HD. I confirm all of the above statements, except I dont know about a beefier Alternator.
Mainly: .15 thick frame rails, 4.10 Sterling rear end, trans cooler (super cooling).
I 'd recommend a factory tech separator plate. A common factory trick to get a larger towing capacity, is to slip the primary and secondary bands. My '03 was set-up that way, and its trans went just before warranty.
Get the trans to 'lock out' in every gear, then work on motor torque.