The 5531 should work great with your truck and the engines listed. I have a 5831C in my 46 2-ton. It gives a bit more usable speed on the highway, and has improved the performance in traffic, especially with a load on the back. Also, without a load, it has allowed me to start from a stop in 3rd under, giving a good burst of speed for merging into traffic. As Grigg said, check the ratios in yours and make sure you have some that jive with your driving goals.
I posted awhile back a thread detailing some of my driving experiences. The thread was titled Auxiliary Transmission Ride Along.
