My 67 GTA has a system (well at least part of) from NPD and it is actually a very close to original system. Only problem I had was the "H" pipe was welded too close together for it to mate up with the manifolds correctly, so I had to have a muffler shop cut them and splice in an extension piece so it could fit. Otherwise the hangers, and all other mandrel bent pipe was very good, quality. A+ repo product. I can attest to the transverse, the h pipe section minus the little speed bump i had, and all bracketry, and hangers... everything else i pieced together from NOS stuff from various swap meets.
If you are going with an aftermarkert type system as opposed to the "stock" then yes aluminum, or titanium, etc..If you go stainless, make sure on the specs its at least 409L type which is the best against salts, and oxidation and humidity. Aside from popular belief, just because something is "stainless" does not mean it will not inhibit corrosion, or other types of "rot" (i install fire sprinkler systems for a living, and trust me stainless is thought to be so "great", but just leave the pipe or even the special heads in a humid place like a pool, and after a few years it looks worse then steel) but you are gonna pay handsomely for it. Just a forewarning, make sure when buying any aluminized exhaust, that the pipe is "rolled aluminum" not "dipped aluminum" the dipped stuff will burn off in a few years and then it was all for nothing whereas the rolled type is actual milled aluminum, not just steel with a "hot dip coating".