My father (Beaubeau) needs to keep in mind that you should NOT tow in overdrive- the gears are much weaker. He wouldn't have had a problem if he had stayed in 4th. Of course it won't be an issue in my 7500 lb. truck, even with a 10k lb. trailer. But in a 13k lb. truck with a 20k lb. load behind it...4th would have been wise.
Here is another question- why not just use the Clark tranny? Other than not being synchro'd and possibly not having OD (depending on model), it would be a great alternative to the Spicer. For an off road vehicle, or for heavy use, OD probably wouldn't be missed much.
Jim