Sadly, I think there is just no hope for the auto tranny unless you send it somewhere to have it built. Like I said with the w58's, it's kind of hit or miss with them. I know a guy that broke 2 @ 440whp, and others that have broken them with less, and another guy that said he made over 700 with his. His is reputable too. It finally did give out on him too though, but it lasted wayyy longer than anyone else's. It's all about how you shift though. You pretty much nailed the swap down, also get a new slave cylinder, go ahead and replace the line from the clutch master cylinder to the slave cylinder with a SS braided one since it's only like $45. You can definately find w58's for only a couple hundred dollars though.
Another alternative you might want to look into is doing the r154 swap. More expensive than the w58, but can hold a lot more horsepower, and it's definately cheaper than the v160 swap! Plus with all these mkiii people that you probably know, you could snag one for cheap, then it might not even cost more than the w58! You gotta have a 1JZ bellhousing though!
