Re: what exhaust is loud on a 79 f350
You can use glasspacks but keep in mind, the shorter the glasspack the louder it will be. I would use 18 inch glasspacks. The 24 inch and larger are just too quiet and the 12 inch are too loud and high pitched. If it were me I would buy the headers and the reducer's. bolt them on to the collectors (the ring at the bottom of the header that bolt together), install them and have a muffler shop custom build from the reducers back. Glasspacks are pretty much the same and I would imagine you can same some $$ buy getting them from your local muffler shop.
Have you thought about looking into flowmaster mufflers? They would sound great with some turn downs right before the rearend. With 3 inch pipe those would rumble. Go to youtube and view some vid's using Flowmaster Exhaust as keywords.