The last time I did Mount Diablo, I did it on a 29er with 125mm cranks, after using 150mm for my previous attempts. I beat my previous fastest time by five minutes, and was the only one in the group to make it up the extremely steep (14%) bit at the top.
Diablo sounds a little less steep than Whiteface; it’s something like 11 miles of climb for 3300 meters. But I wouldn’t worry about the shorter cranks; if you’re trained up with them, you’ll do fine.