I'm not surprised on the rust. These cars were just driven new.
Seemingly all European cars of the era had little or no consideration for protecting the tub for anything really past 5 years. Porsche is included in that list.
Anti-corrosion ideas in my opinion are actually something coming out of US cars. Laugh at that if you want to.
Japanese cars are also incredible rust buckets as well from the times.
The Japanese ironically called the solution, the "not invented here" solution.
What they mean by that is that eventually they accepted the idea that they would have to use others ideas and proven procedures even though they did not invent it. WITHOUT CONSCIOUS of stealing the idea.
We here in the US for a long time felt that the Japanese success was because they took our ideas, stole them as it were, and improved and applied them to their products.
All of the Europeans positively refused to do that. Ever try telling the Italians, or any other European what or how it should be done?
You shouldn't if you value your physical well being, your families safety or a general tar and feathering. I guess that's an American thing though? Maybe being pelted with empty Cinzano wine bottles filled with gasolene? LOL!
That I used to see from the "University students" on the "left bank" in Paris around 1970. The US Embassy was often a target of that, but I digress.
Looking at Detomaso cars. He actually really tried really hard to protect these things with about a hundred pounds of tar based undercoating. Unfortunately the concept of vat dipping the tub in electrostatic primer never caught on there at all. It is what it is though.
I personally have found that it is best to leave the windshield in place.
They are really touchy and crack very easily.
There are actually aftermarket windshields available here in the US, or at least recently were, by two different glass companies, cheap.
Try $275 each.
Problem is 1) they are aftermarket glass 2) they are thinner than the originals.
LAST time I looked, originals were about $1,200 each new. That was a while ago. I'm afraid to look now.
In an perfect Universe it would be best to remove the glass, strip off all of the paint and reprime, reseal and repaint.
In reality, you can leave it in place and run the new paint right up to the gasket and have it virtually undetectable.
When working on any of these cars it is difficult to know when to end your own personal degree of perfectionism and just learn to compromise?
In my case I have often found out this can be the cause of why what was supposed to be a 10 day project turned into three months or more.
I've been doing this stuff for over 40 years and you would have thought I would have learned better by now? That can't be considered a personality attribute on me?
It's like worrying about your own personal possible medical issues, or developing them. You have to stop the paranoia for something that you can't control and just deal with everything the best you can when it is staring you in the face.
As James Bond was quoted, "you only live twice. Once when you are born, and once when you stare death in the face". (<...and survive it I presume?)
Maybe for me looking at rust and/or seriously twisted metal is the same thing? People like Johnny Woods are offering new injections of faith and hope?
Best of luck on your project...and keeping it away from your daughter who is already claiming it as her's!