Hey, I haven't had a chance to read the entire build thread but looks good from what I've had the time to read!
Judging from the pictures of the tank (I'm assuming it's the one you were talking about) that should be a pretty simple fix as long as the metal bordering the rip is in good condition. I already said it in my build thread but thought I'd pop over and toss it out here, if you want to send me the tank I'd be happy to fix it, it'd just cost you the price of shipping both ways. If it's just the straight tear I'm pretty sure (like 95%) that I can get it fixed and air tight without any sort of tank sealant (although I still like using them, I'm just wierd, I like to have all the holes fixed before I seal the tank). You'd likely have to use a wee but of body filler when I'm done, but the tank would be useable. In order to weld on it I'll have to flush it with water, so there will likely be rust inside the tank when I'm done (One of these days I'm going to get CO2 to use to purge the tank, but for now, water will have to do the trick.)
Anyhow, shoot me a PM and let me know if you're interested. Oh, and the tank is steel right? Sadly, I don't have the ability to work in aluminum yet, some day though.
Brandon