I purchased a 71 914 from Florida
Got it up to Ohio, Put in my garage, opening & closing driver door rubbed on door catch. I put jack just behind door, made a differance, how scard should i be

Take the rocker panel off and see what you got.

You may want to think about learning to weld....

Probably your best bet would be to have some of Brad Mayeur's chassis stiffening kit put on the car. Not concours, but a great way of shoring up a weak chassis. Could be a DIY install, but if you have any doubts of your ability, have a pro install it.

That kind of flexing on the passenger side is common w/ rust, time, accidents, etc... Having it on the drivers side is not common & if it's the same cause then parts will be difficult to obtain. You need surgery, & I'm willing to bet you have cancer (rust). All is not lost, for this is a car & not a person... a car can be made to live forever. Might have to get yourself a parts car. Just takes $$$!
Good luck!