Back in the day everything was repairable and today it still is if you are not following ASE or manufacturer standards. We've all heard of the guy at the shop that can align any vehicle, but he used a bowl torch and a hammer instead of replacing parts the correct way. The collision repair industry has a come a long way with regulations, some unfortunately due to law suits.
When it comes to classic cars and older vehicles without parts availability, well those rules go out the window, and you have to use an experienced tech like your father.
I-CAR, ASE, and many manufacturers don't allow you to repair folded over metal or severely bent rails. There are temperature limits as well often measured with heat sensitive crayons. It's been a long time since I've seen anyone doing repairs like that. Easier to replace than to have the liability of a frame cracking.
"The following are just a few examples of recommendations found in vehicle-specific BRMs:
- According to the GM service site, for ultra-high-strength steel (UHSS), dual phase steel, and ultra-high-strength dual phase steel, "The use of heat to repair damage is not recommended for this type of steel."
- For mild steel and high-strength low alloy (HSLA) steel, GM specifies that, "Controlled use of heat can be used to repair damage, if the heat does not exceed 650°C (1200°F). The heat should be applied a maximum of 2 times, for up to 90 seconds."
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Recommended GM Steel Repairability Matrix.
I-CAR REPAIRABILITY TECHNICAL SUPPORT PORTAL
rts.i-car.com
I assume that the entire ladder frame is no longer straight and in more of a trapezoid shape. Imagine trying to align the suspension on that.
Sure you can pull one side very hard but then will the cross sections, rivets, and frame be loose? If the frame was off a cm on one side that is technically repairable by some standards and you can get away with it but I wouldn't want that on my vehicle. This frame most likely had the trifecta of frame replacement damage, mashed rail end (requiring a new frame horn) swayed rails (towards driver side), and trapezoid frame (where the rear bumper is no longer parallel with the tailgate).
I would make sure 100% that a new frame is coming from the factory and not a LKQ one because, who knows what happened to the LKQ vehicle? A used frame would have to be remeasured to make sure it is true.