Originally Posted by Mutt's37Buick
The book "Hot Rod Builders & Chassis Builders Guide" says "some modern high performance frames are made of high tensile steel that does not respond well to being cut or welded." I would not expect that a 03/04 Crown Vict would have a chassis made of this high tensile steel.
1) Can anyone confirm this is the case?
The only thing I can confirm is that nothing on a swap responds well. Every last piece has to be beaten or heated or twisted or ground or filed into submission. If I were doing what you're doing, I'd proceed as I have on every swap I ever did and include additional material to strengthen the entire structure.