I've heard rumors that
one of the "high flow true dual" headers (split manifolds, as true as it gets without a "V") on the market for the 258 will fit, but I'm not sure which one it was.
That being said, I can tell you for certain that it's a very tight squeeze as as is, there's not much room with the crossmember and front diff right there...
Also keep in mind most places in the US, even emissions free places, require exhaust to extend "beyond the cab"... so in a truck, side exhaust is fine, but with sedans and wagons it must go all the way back.
I'm also redoing my exhaust right now though, so let me know if you find a header that will fit.
I went with a flowmaster 40 (vanilla, not super or delta) single in dual output.
I saw this video and liked the sound of it:
http://www.youtube.com/watch?v=PJasB7typfwHe's running the single header, to a flowmaster 40 single in/out, with a "y" pipe, but I decided I wanted the outs to be the same length, and if I want to mount it where the old muffler went, I might as well just have them protrude side by side instead of opposite sides of the gas tank.
I'll post some pictures when it's all done.
As for the headers, here's some more discussion on the topic:
http://forums.amceaglenest.com/index.php?topic=38616.15