I've never known "what did you do" to be contracted in writing.
**Are you sure? Did you notice that "used to" was already used in the sentence preceding the first blank? Although I think it's still more acceptable in the first blank than in the second one**
Yes, I did notice first time round but on reflection and with a wee bit more time to spare I would say you are correct. "Would" in both gaps does roll off the tongue more easily in speech.