Arm length usually is better for OT's and DE's. Guards and Centers double team a lot, so it's not as big of a deal.
And 33in for someone 6'4 might be a hair short (they're about the length of my arms and I"m 6'2.. my track coach was hoping I'd have 34" arms to be better at discus). But I wouldn't call them short arms.
The real question is what is his hip height, and if he has good knee bend. Don't want a center who's too tall or plays with high hips. Want to match up with the QB.