Hm interesting. Im about to "create" an exhaust for my nas replica project. It has a 3.5 v8 fitted. The manifolds are 3.9 efi twin outlet , these will then go into efi twin downpipes and standard y pipe. then from the back it will have a standard td5 rear silencer, small box but huge bore, then a td5 mid box replacement pipe and then some how join this to the y pipe. Should sound nice !!! and be made of easyly available parts.