Any ideas what may cause torn inner fender well, popping by back door

[b569Charger]

That's called massive torque in one of the original uni-body designs. Pretty common problem actually.
Weld in a set of frame rail connectors from US Car Tools then weld in some additional reinforcement behind the panels where they cracked.
http://store.uscartool.com/66-70-B-Body-frame-connectors.html
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What do you think of the Mopar brand bolt on frame rail connectors? Maybe not as good as the weld kind, but would it help out? I might be able to handle that, and I don't have the torque boxes by the back wheels, either. If you get frame rail connectors, are the toque boxes (hemi, convertible car style) also needed? thanks!

 

[Runner 68]

What do you think of the Mopar brand bolt on frame rail connectors? Maybe not as good as the weld kind, but would it help out? I might be able to handle that, and I don't have the torque boxes by the back wheels, either. If you get frame rail connectors, are the toque boxes (hemi, convertible car style) also needed? thanks!

I'm no expert on this subject. There are several threads on FBBO where those far more knowledgeable than I am have given their opinions.
From a logical stand point I think weld in is better but bolt in will still probably solve the problem. I pondered the difference for a while as the weld in option is definitely a more permanent approach.
I think the torque boxes are only needed if you plan on drag racing or hammering the **** out of it on a regular basis.
I'm sure others with more knowledge than I will chime in. That's whats great about FBBO.

 

[b569Charger]

This crack is pretty common on Chargers. On the cars I've had over the years, mine have all been on the driver's side. The fenderwell crack is a new one on me.



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OK, I think I got the picture now

The fender well crack does indeed seem to be a thin layer of bondo. It looks like in a past life a very old repair was done
http://www.autometaldirect.com/1967...dy-front-inner-fender---pair-md-38-p-436.html

I jacked the car up and did a long study of both sides of the car to compare them, and how they have been attached/ welded. It seems like a replacement inner fender part it was put on decent, but my guess is they wanted to smooth out some imperfections. Based on the colors and patina, it looks like it was done long ago. Frame rails look fine, straight, etc but we will see when we take it in after our front end rebuild is finished, but a few older smart car guys have put it up on jack stands and looked it over, seems they all think it's good