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You can simply use "pure" titanium which is Grade 1 and that will work fine and is weldable to the existing titanium parts. The bigger problem is getting it welded and to do so you need to shield it with an inert gas so that you don't have problems with the weld. A shop experienced with welding Ti should be able to handle it.
Look for a salvaged Ti Pipe off a wrecked C5 Z.
If the pipes were damaged in 'a wreck', they're unusuable as a Cat-back, but would allow you to cut pieces you need for the Ti Mod.
Call Marc at VetteNuts, and start searching there. He may have just what you need. They are a Forum Vender.
Google search for "Titanium Joe" to get the pipe, as far as welding goes it's not necessary to shield it etc as not a stress nor loaded area, buddy at work stick welded mine with the proper rod and low heat.... No issues ....3+ years old now....titanium exhaust are no longer uncommon on high end cars, some exhaust shops are familiar with it too...