On the Gathering, they used Db or C#, however you wanna call it, if I'm not mistaken. As for their older stuff, it's quite possible they used Eb, but tuned the frequency down to 435. Old Machine Head albums were like this, which is difficult to get the right intonation if you don't know all the info.
My advice is just to try different tunings, and if that still doesn't work, then try at 435 instead of 440. I think Testament typically tuned a half a step down, though, whether they were in D or E...except for Demonic, which most of them were dropped tunings. Blech...I hate drop tunings.