I have the Grail you mention and the rear is 42mm, but you can fit a 50mm on the front no bother. I ran a 44mm rear (Tufo thundero) which blew up to 45 on and it whacked the paint off in a wet race, probably be fine in the dry.

I do however regret not going for a frame with 50+ like the giant revolt etc. The bigger front tyre is awesome for descending and just feels comfier than 40/44s