for a gt just go with 3 inch PVC piping.
smoked a couple of rice bunners tonight it was fun.
just an fyi with pvc pipe. when it gets hot it will give some. even if you clamp the hell out of it, it will still come loose eventually. when i pulled it apart the end was clearly bent in. might wanna use some sealer or something along with clamping it.
please tell me that you went with the 4".i just got it done tonight it sounds bad ass. smoked a couple of rice bunners tonight it was fun.