4oo cfs seems considerably low for that section of river.
I've been down there with a 13 footer when the gauge
read just around 800 and it was very shallow and we did
drag in several places. I'm sure it's doable, but honestly
I probably would opt out cause I would be worried about boat abuse.
With regard to rapids, everything is read and run (in my opinion).
The only 'real' rapid is Eight Foot rapid and it's a cinch, just hug
the wall on river right, and then make your way back center to
avoid some big holes. The only rapids that become more dicey
at lower levels are downstream of mexican hat, and since your
not paddling that section, there of no concern.
Ps. bring your bug juice, and a portable AC if you have one.
It's freakin hot on the San Juan!!!!