If it worked with the 350 but not with the 400, I'd say the problem is probably with the 400 chip rather than the ZIF or the heatsink. I suppose it could be a problem, but not likely since you already have additional heatsink fans.

[EDIT] Really, you may as well get a Sawtooth on eBay--they're dirt cheap these days. I have a 500MHz Sawtooth that I absolutely love.