Hi AnnMarie,
I would have the whole cloth piece framed. I would not cut up the piece at all. I would not use it as a mat either. You want to be able to see all of the different pieces of fabric.
There is uv protection glass and you can have a small shadow box frame and need to use acid-free paper and matting. Your local Hobby Lobby and frame stores can give you suggestions. I would check out several with their pricing. There are some that let you frame your own.
What a wonderful heirloom that's probably from past family members. The wedding dress is an heirloom as well. There are places would love to have such as item if you really don't want to keep it in the family. Do you know any of the history of the dress? Handle it with care and don't do anything to it until you talk with an expert. Keeping it intact without cleaning keeps the value and integrity of the dress.
Good luck with your treasures.
Pam M