Can’t find the exact docs on which Digital pins are 5v tolerant but @zach did mentioned here:
But TX & RX is mentioned here…
So looking at: http://docs.spark.io/#/shields/shield-shield,
It uses a TXB0108PWR as the logic converter. I think others like:
works well too 
If you want to test stuff out 1st, you can use a simple voltage divider 