Still way over geared. Use the calculator and don't expect that setup to work reliably until it's geared for around 40 mph, maybe even less for 4s in a truck that big. At the moment your geared for around 70mph (if you are running a center diff with no trans and the 7.5" tires). The last thing I would do is try and use that setup on 6s, you'll be geared for 100mph and will have a similar result to the first time.
Will this spur work in your truck?
http://www.amainhobbies.com/product_...ducts_id/11243 If it does, you should be able to fit smaller pinions. With that spur a 10t pinion will give around 36mph and an 11t around 40mph. I'd start with the 10t and see how it goes. May even need to go down to 9t not sure...it's a big truck.
Like some of the other guys said, the best setup would be to go for a slightly bigger motor with lower kv and use 6s, while still gearing in the 40mph area. Smaller tires will help as well, but I guess it would kind off defeat the purpose of the truck.