I'll toss in another vote for the 9XL. Even the 8XL will get too hot before you're ready to give it a rest. In a big truck, the MM likes the higher turn motor too.

I've been running a MM with a 9XL in my Monster GT for some time now and without issues. I am getting ready to upgrade to a Compro/Neu combo, but that's just because I've become spoiled. The newer Compros are pretty darned smooth and the performance of the Neu motors is undenyable.