Can an old chassis be restored?

Of course, it can. But usually it does not make any sense since replacements are easily available. To do a real restoration on the frame the body must be taken off as well as motor and transmission so the chassis can be turned upside down. Then the panel underneath must be cut out to be able to reach the support structure inside which always is partly rusted away. This inner structure gives the chassis it's strength so it must also be restored carefully! Do not forget to prime and paint. Afterwards a new panel must be put underneath and welded properly to the internal support structure and the sides. Finally the chassis must be treated inside and out with a good grease based rust protection or all the work will be worthless within a year ...!

This sounds doable but we only did it once to satisfy our curiosity and to learn. It took so much time that we never even considered doing it again!
If you have to pay someone to do such a job the cost for labor counted by the hour will always be way more than simply buy a brand new chassis! Even if you can do the job yourself it pays to go for a new frame since a new frame is always better than a repaired one and it saves you a hell of a job.
What workshops do to make the cars pass the required technical inspections in Europe is simple: They only weld a piece of sheet metal over the rusted area to cover up the mess. Since those inspections only include a visual check for rust even unsafe cars pass without problems if the rust cannot be seen. So better be extremely suspicious with welded frames!

We have seen many different solutions how to weld and reinforce a rusted frame and many of those even passed the inspections without trouble! At some cars you could hardly see the welded areas at all. But most of them are unsafe and only good for maybe a year or two - just enough time to sell the car.

Do the "Corn Flakes Test"! All you have to do is to smack the frame from underneath with the palm of your hand. If the frame has severe rust inside usually there will be plenty of loose rust particles. By smacking the underside of the frame these particles are kicked up and fall down again. This sounds like corn flakes falling in the bowl. This simple test can identify at least those rusted frames where the welder did not even care to get the rusted particles out before covering the mess. However it is not 100% reliable. If the frame was treated with waxoil after the cheesy repair, all loose rust is glued down from the wax.