LLY's had cooling problems, LB7 and LBZ have never that I have heard. Have you tried the synthetic Evans Coolant ????????
http://www.evanscooling.com/html/npgPls.htm
Not cheap, but works VERY well .....
Yes, they did. I used to pull my 34 cat and behind me was a friend in his 30 deckboat, both Magics. I had the dually LBZ, he had a 2500HD LLY.
His load was lighter, but my LBZ would pull on him and stay cool, where his LLY would start to get hot and he would have to get off of the throttle.