Be sure and check to see which version of code your municipality follows. In Tuscaloosa you can have a flush floor hearth extension 60” X 20” but it must have adhere to a specified R factor. There is a product called Micore 300 that carries a very high R factor that can help achieve your goal. You would install it below the non combustible hearth extension.
The mantel height required is 12” above the fireplace opening for masonry fireplaces (in Tuscaloosa). Most municipalities follow manufacturer’s guidelines for pre fabricated fireplaces. Most of those guidelines are more restrictive than masonry fireplaces. Be sure to review the installer’s guide for any pre fab fireplace regarding clearance requirements. They are likely different for each manufacturer.