You don't need 500hp, and if you think you do, you're kidding yourself. The new trucks with 440hp sound like they should be tire shredding beasts, but they really aren't. and while they have 440hp, it's not the same as if you build a 440hp 7.3L, it's a different kind of power. It's smooth and quiet and refined, you'll never get that from a 7.3L on a good day. The advantage of a new truck is refinement, not power.
If the trailer is too big, the truck can't pull the load, and you need a truck set up differently, then buy a newer truck, or set up your truck for the load. The problem with essentially doubling the poser generated by a 7.3, you need a transmission that is capable, you need added cooling capacity, there is more to it than building a 500hp engine. You need to regear or add an under drive or both, it's just not that simple, and what it would cost to do it right, you might as well buy a newer truck.
Better option is buy a nice used truck that's more capable, but not $60K+ like a 2008-2012 model. And don't be stubborn and try and be brand loyal, go drive a dozen trucks, set up with different options from Ford GM and Ram, you'll find that one of them will be a better fit to your needs than all the others, and that's the one you buy.
The Current Truck (Oct-2013 to Present):
1999 (Early) F-350 Lariat, CCLB, SRW, Black/Tan
7.3L Powerstroke, 4R100, 4x4, 3.73, 37x13.50-18s, '05 Axle Swap, 6" Fabtech lift, GTP38 "D66" turbo upgrade, SCT 4-position chip with custom tuning by me, Straight Piped 4" down pipe through 5" dual stacks, N-Fab full length triple steps, Pro-Tech cab guard and tool box, louvered 5th wheel gate, Hella 500 Fog and Driving lamps, much more to come...
The one I'll miss, (Mar-2009 to Jan-2014):
2000 Excursion XLT 4x4
6.8L V-10, 4R100, 4.30, 285/75-18, 271whp / 388wtq
The one I won't, (Jul-2005 to Sep-2012):
2005 F-350 XLT, CCLB, SRW, 4x4 FX4
6.0L PSD, ZF-6S650, 3.73, 35x12.5-18, 4.5" Lift