I totally agree w/ FltTest. Something definitely sounds fishy. There's no reason no to use new parts - especially if it will void part of your warranty.
I would definitely consult an attorney. In addition, I'd have a discussion w/ your insurance agent, especially if you've been with them for a fairly long period of time & have no other claims. They may help you understand what's going on.
Also speak with multiple shops to get their professional opinions.
Lastly, DO NOT settle w/ them. Continue to say no it's not acceptable until they begin to give in. It may cost you some $ for extended rental car time, but I wouldn't accept what they're trying to push on you...these trucks are very expensive & yours needs to be made right. If you weren't at fault, the other insurance company should bend over backwards for you.
You gotta be aggressive & not accept anything, until you're fully satisfied...even then, if the truck isn't 100%, don't accept it from the repair shop. When you get it back, it should be 100% they way it was prior to the wreck.
I hate these f'in insurance companies - auto, home, health, life...it's all a racket. Allstate has plenty of money!
Sorry for the tirade!!!!

This subject pisses me off to no end!
Tom