Here's a little dyno check list:
1) - Make sure the tires are inflated properly. Low pressure increases rolling resistance and lowers power numbers.
2) - Make sure the dyno operator runs your truck in Drive or your 1:1 gear. If you have a programmer, or if you are having tuning done, make sure the converter is locked and the truck doesn't downshift.
3) - My experience is that load dynos are better only because they light the turbo at lower rpm and give you more representative data. But make sure that the dnyo shop you use is familiar with diesels otherwise it could end up being an expensive waste of time.
4) - Finally, run the truck with th same tun you drive it around with the most. That's the tune you want to know the power ratings for. Race tunes, Looney tunes, Insanity tunes, whatever you want to call them, they migh lay down a big number, but if you never go to the track or sled pull, then those tunes are just feel good numbers for when you dyno and are otherwise impractical.
I've probably made close to 200 pulls on the dyno with my truck now, it's never hurt the truck, so don't be affraid to do it.
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