It will use textures from loads of different stages (as is the AMP way) but will mostly revolve around MM3DW1 and Toad Man.
I vote for Underground Drains since it sounds cool. I would like to help you in this project so you'd get work done a lot faster, however I'm not sure I have much time for things like these anymore after I finish my CTF map and help my bro with his MM3TOPX. (Storm Drain sounds suspiciously like Pokemon. >_>)
But if you need me for anything like music compression or something I can help, send me a PM. I'll do my best.