There will be a few issues with regards to putting dual exhaust in your 1980. I'll start near the front of the car and work back for the purposes of this discussion.
First you will need to replace the stock tranny crossmember with a dual hump cross member from a 85-93 5.0. The 1980 single hump crossmember will not accomodate any of the late-model H-pipes out there. In order to mount it you may have to get the brackets rewelded as the mounts are in a slightly different location (not a big deal, really). You can get a custom H-pipe bent to go under the stock crossmember, but ground clearance is seriously compromised and you stand a good chance of mashing one of the pipes over a speed bump.
Your second problem will be with routing the passenger's side pipe over the axle tube and along the side of the fuel tank. The 1980-84 Mustangs had the rear brake hose connections on the passengers side of the axle, and that can put it too close to the exhaust if routed incorrectly. This will mean that some custom bending of the tubes may be necessary, or you could switch over to 85-93 lines which connect closer to the center of the differential.
In addition the older tanks are mounted slightly to the passengers side when compared to the later cars with dual exhaust. This can make things pretty tight, and 2.5" pipes may not be possible without interference with the rear tires (depending on their width). With my first 79 that I put duals on we had to go down to 2.25" pipes, and even that was very tight.
Hangers are not really a problem, as any good muffler shop will be able to put them in to hang the pipes.
I don't mean to worry you with any of this, but I wanted to let you know some of the things that you might encounter. A good custom exhaust shop should be able to work around these things and get you set up, but you will save yourself a lot of hassle by doing at least the tranny crossmember swap.
Good luck, and let me know if you have any questions.
M&M Member #395
'79 Mustang Indy 500 Pace Car
Ian's Pace Car Website
Vancouver Island Mustang Association Website
Indy Pace Car Registry of Mustangs Official Website
"If you get to thinking you are someone of some importance and influence, just try ordering another persons dog around." - Author Unknown
[This message has been edited by Gearhead (edited 06-11-2001).]