This is the only place I have found that has anything for our series of cars. They are listed for a 73-77 Chevelle and only at 2-1/4" or 2-1/2". Maybe they can make a set at 3" for you? Possibly worth a call. Either way I hope this helps.
AJ
http://www.heartthrobexhaust.com/mandrelbentpipes.htm