as said above buy a dual exhaust adapter then a GT catback should work once you pick up a set of exhaust hangers that you are missing
the issue with just having the Y pipe cut off and true duals welded on is just that... welding them on means they cant easily be unbolted to access the transmission. If you need to get at it you will have to cut the pipe off. They may can weld on flanges and bolt/unbolt the exhaust. They should use stainless steel ONLY in 2.25 inch diam pipe custom bent of course and make sure they clear the axle area high and wide for future lowering plans
some ***** about the gas lines, GT's run their pipes the same way so no issues there as the pipes will be somewhat close to the gas lines.