I am hopefully going to be doing a custom exaust for my 1980 X and have been looking a bit and talking a bit. I think I will go headers to a glass pack, because a glass pack is light and cheap compared to a supertrapp or flowmaster. I use the car 90% for autocross and that gives me what I need.
I do not know of all the options available for FIAT but it is going to depend on what you are using the car for, and if you need to pass any emission testings with it or not. If you are looking for a better RPM response (quicker acceleration, use if autocrossing) probably just go with some sort of glass pack, or maybe a supertrapp, they are very low flow resistance, but obviously can be quite loud. If you are looking for just a good solid power band for all your driving needs (if you are road racing or hwy crusing) you will want something that gives a bit more back pressure to the engine like a flow master, or a custom tuned muffler.
Either way you may want to call up your local custom exhaust guy and just chat to see what they recommed for your application. Good luck, and let us know what you come up with.
Also where are you planning on getting your headers from? And what did they run you?
KT
1980 Bertone
99 DSP
1980 X-1/9 Black
"Roxanne"