Welcome to the site Grey..
The prior posts address some good very common issues you should be looking for on that 70'.
As far as anything that might be a deal breaker by making the car failing to have integrity anymore, I guess that all depends the level of your skills, your time and the money you have to spend.
A major one as indicated in a prior post is the frame rails. They like to rot, especially in the back. They also like to rot from the inside out. Changing these out requires more surgery than a simple panel replacement or patch job. Having some experienced help and/or prior experience yourself is typically a must. That is, if you're going to try and change them on your own.
Torsion bar crossmember to inner rocker junction. These B-Bodies love to rot out here. It is unfortunatly a bad design that creates the perfect area for dirt/dust/grime/grease to settle and start rusting out. All the B-bodies i've bought up here in the great white north have been rotton in this area. This area, like the frame rails, is a critical part of the unibody structure as well as the suspension and requires a bit of advanced metal/fab experience to replace. They do make repair caps to slide over the typical corrosion area instead of replacing the whole x-member, but still require welding/fab work.
Rear torque box/leaf spring front shackle area. This area is just forward of where your leaf springs mount on the front side. Supports and distributes quite a bit of the stress/torque rolling through the body. Also a pain to change out, but you don't see them rotting our nearly as much as the prior items listed. Either way, integral to the unibody..
Inner/outer roof......
It is ALOT of labor intensive work to remove/replace these. It is also a pain in the rear to try and fill/smooth over any dents and repairs, being they buckle in and out so easily with a small amount of pressure. Be prepared for a lot of work/time if you plan on taking one of these on.
Some of the areas typical to rot, but not necessarily key to "core of the car".
-outer wheel houses at mate up to quarter
-inner wheel house at mate up to trunk pan
-rear crossmember
-trunk gutters to quarters
-trunk extensions
-rear window lower window corners
-quarter panel all around wheel house
-outer rockers and certain areas on the inner
-lower fender dogleg on backside of wheel openings
-lower 3" of the doors
-rear door jam in bottom lower corners
-Door hinge pillar, at the hinges as well as the front lower side of the pillar
-Trunk deck lids, across bottom lip
-rear fill panel and fill panel to trunk pan mate up area
-Cowl, if there is carpet installed and has mildew/mold or is wet/moist in the front floor pan areas, typically that is a good indicator the pleneums on the inner cowl are shot-tough fix. I've seen people even put duct tape over the vents thinking it would help.
-lower corners of front winshield
-under the hood, around shock towers on inner fenders as well as hood hinge mount areas on inner fenders
-battery tray area
And like mentioned before, the floor pans as well as the trunk pan.
These are the typical areas you find rust, but are no means the only places you can find problems. It varies wildly on how/where the car lived it's life and how the prior owners took care of her. Do not rely on the seller's word or pictures to get a feel for the cars condition. Put you own two eyes on it and check every square inch. Try to remeber that what you see on the surface can be just as bad if not worse underneath
The good news is that almost every single area I mentioned you can get replacement sheetmetal for. The very few areas not being RMFG, you can get from a donor or have a parts car. The flip side is trying to figure out if you can balance the skills, time, money, facility and planning to do it.
Before anyone one goes and makes an investment like this, you really need to sit down and take a good hard look at your budget, skills, time, money and lifestyle. It is a major major major undertaking to restore a car, even if you're not going to be the one doing it. Just coordinating a resto can be stressful. We see a lot of folks come and go on here that didn't put their ideas together and plans on paper before jumping on such a task and end up loosing money as well as time when they finally sell the car off in parts/pieces or pennies on the dollar after all the headaches and life's constraints finally took their toll. Even if the car is not needing any sort of major resto, just the time and money to maintain, house and drive these old cars can weigh heavy on a person. By no means am I trying to discourage you from buying your car, I think everyone should own one..lol They're great cars!! Again, they are a lifestyle change..Good/bad, depends on you and how you approach this investment/hobby. I don't know you personally or your knowledge/experience with these cars....... So sorry if this all comes off brash or blunt.
Wish you the best of luck and hopefully this little novel gave you some sort of insight into your question.
-prop
im not a huge expert or anything but my 2 year car search led to me seeing many rusty floors in the late 60s early 70s b bodies, so watch for that nasty rust, the rest can be more easily replaced, if its more solid of a project go for it, i hope you have fun if you get the car, good luck!
