I gave one of those monstrosities away a couple months ago. I really doubt it will fit in a rack chassis of any type. You could always mod something but is it even worth it to do so? Why not just get a rack shelf and plop the tower on there on its side?
Thanks for the reply. While I accept that the motherboard likely won't fit in a standard rack chassis, I was hoping it would fit in a Dell rack chassis of the same era, if I can find one floating around. If not, I'll just dump it and build a server with the remaining parts. The PE1900 chassis is going to the recyclers though, one way or another.