Per my prior e-mail exchanges with AJP, it was suggested to me to use a gear that allows a pull over 10-15 seconds. Longer than that, you are putting unneeded strain on the PT.
I ran my last round of Explorer dyno testing in 3rd gear, and I was way toward the bottom end of that range (maybe even 9 sec, I'd have to double check). I did something like 20 pulls, though, and wanted to be extra cautious not to over-strain my truck. I also did all my on-road data logs in 3rd gear, so I wanted to have a comparison to my dyno pulls.
Traditional inertia dynos calculate force (F=ma) using the known mass of the rollers and the acceleration of the vehicle, which is why it's important to try and run as close to 1:1 gear as possible to minimize drivetrain losses and errors from correction factors. Load based dynos (like the Mustang I ran on) uses a load cell to measure roll force directly, so gearing doesn't really matter.
You mentioned that the DJ you'll be on has a load cell as well, so maybe gearing doesn't matter as much? I think the most important thing is to run all YOUR pulls in the same gear (now and in the future). In that case, even if your absolute numbers are off a little, your relative gains/measurements will be accurate.
Couple other thoughts based on my recent experience:
1) Have them target your pulls from 2000 - 6000 RPM
2) If you decide to data log, I found it much easier to just use the X4, and I loaded the data onto my laptop between pulls. I originally tried to log directly to the laptop but had persistent connectivity issues with all the ignition cycling.
3) Run with the hood up. Most dyno shops don't have high velocity fans, and there's no sense in heat soaking your engine. Hood up will allow intake temps as close to ambient as possible (more representative of real-world driving) and not overcook your engine bay due to otherwise lower air velocities.
4) My XSport has a cooling duct under the right side of the engine that runs to the rear turbo. I brought my own separate blower fan to blow air down this cooling duct, since the shop fan didn't direct much air there. Not sure if the SHO has something similar or not.
5) I monitored CHT temps and did not start my next run until temps were back down to where we started the prior pull.
Have fun!!
