When you call ATI, ask to speak to Jim Batey. He's the owner. If he's not available I think the next guy you need to try to talk to is Paul, but I'm not sure on the second one.
They'll want all sorts of specs and info. They'll probably tell you that they don't go by "stall speed". I know their converters work 'cause I've used them for years, and I've seen 'em produce results. There are other great converter builders out there. I've used several different brands, but I always go back to ATI.
