You cant pull anything back out to perfection. Once crumpled, always crumpled, and pulled back out the crumpled structure will always be some percentage weaker than before it was crumpled.

Whether a tree, several trees (BTDT), or a rack (done that too), when it gets into the heavy structure the best alternative is to remove the damaged structure and section back in undamaged structure.

In any case, its virtually impossible to return a damaged unit body back to "as it was". Thats not to say its weaker, a good body man/welder, can make the structure every bit as strong. But even the very best craftsme, cannot always do such superior work that it can be fully hidden.

Having said all this, it should be noted that metal has memory. Often a good yank in a single plane can often return a damaged area back to where it was, or so close that further pulling will only suffer to do further damage.