Go for it...
Regardless of what the flows are, you should still do this trip. The upper (Sand Island to the Hat) doesn't really matter what the flow is...you can run it under 300, which it won't be.
the lower (Mexican Hat to Clay Hills) is runnable lower than you would think. We did it in duckies, totally self contaned at 370. Now granted it was low, but still do-able.
Even in a raft you are ok wt 600, or 700...just pack accordingly. Possibly bringing less wood, or a medium sized fire-pan, or leaving out a DO or somehting like that if the flow is really low.
I would highly recomend not leaving behind any beer regardless of flow. The last 11 miles are a pain in the ass regardless of level, and you will need that beer!
Have fun, late April is a great time to be on the river. I would plan a night at John's Canyon camp...cross your fingers the waterfall is flowing...what a magnificent sight!