I got the Mustang 'engine and chassis' kit from AMK over ten years ago before they cataloged anything for Cougars. I was very pleased with the quality and the bagging of the various components (makes it almost impossible to screw it up!).
I don't think there are exhaust kits that I know of - the mustang ones come up a bit short - but if you have a tubing stretcher... ;>) you can make it work! I hear good things about AMK products. I may be in the need of a kit from them myself very soon.
Something to ask is the country of origin of the parts in those kits. I bought a similar set for a Galaxie I was restoring several years ago. I was severely disappointed in the quality. Virtually all the screws were made in China and several "self-destructed" upon installation. Retailers for the most part seek out the least expensive supplier. I have not personally dealt with AMK so I cannot speak to their practices.
My personal preference is to re-use original fasteners. I sandblasted nearly every fastener from Isabel, bagged and marked the bags and (Tom at KTL) sent them out for replating. I know I have the right fasteners bcause they are the originals! Not sure what the full kits run, but (besides my labor) this method was cost effective as well.