The 6.4 just wouldn't do it on Colorado mountain passes.
Exactly right. I've tried pulling 8K around with a 5.7 Hemi and aftermarket 4.88 gears (IIRC) all over Colorado, still redlined and got hot in the mountains. I was lucky to do the speed limit in most cases. Nothing can replace a turbo diesel for towing.
12K behind any gasser is going to be limited to short trips and casual use.
