I use a versahual also...mounted to my Fj cruiser. The key was adding airbags to the suspension. However I've not carried a bike over 300lbs.
Frankly I would'nt recommend a Versahaul for a GS800. But if you must, I would suggest adding "Firestone air bags" to your truck suspension for additional stability. They were under $100, and easy to install. Keeps everything level and solid.