I saw a set of sidepipes in a JC Whitney catalog they didn`t need headers, they were 3" with a four into one set-up but only one pipe was open. It looked like you would then hook up this open pipe to your existing downpipes with a bit of flexy pipe. If my memory serves me right they were only a couple of hundred dollars for a pair.
I don`t know what sort of quality the were but the picture looked ok on the web page, not the real thing but I guess they would work ok and they were cheap.
J.