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By "bay" window, I assume you are talking about the "wrap-around" panoramic windows in front?
One major problem is that the body shapes have changed over the years. So, you couldn't buy the window assembly from a modern trailer and slap it into a '60s or 70's trailer because the bodies are wider, and the curves are flatter.
Do a search--I know I have seen at least one other thread discussing the challenges of doing this.
Some suggested alternatives:
1) Buy a vintage Argosy trailer. They were the first models with the wrap-around windows in front (in the 70's). Paint it silver or strip it and polish it (you would still need to paint the steel endcaps), and you would have a vintage Airstream look-alike with with windows you want.
2) Get a 80's or 90's vintage trailer after they started installing the wrap-around windows. Those will come a lot cheaper than a brand new one.