There is a very good article on giving a Model A a 4" chop in the July 2008 issue of Rod and Custom. The chop was performed by Jimmy Shine and there is a wealth of info and lots of pictures. The Model A that was chopped was 5 window coupe and not a sedan, but many of the same principles still apply. You might want to take a look at the article it had alot of nice tips in it.