I believe S9 hash boards are longer. S7LN also has a taller heatsink to fill the internal space and provide better cooling with a slower quieter fan.
Couldn't you just run the S9 with only two boards plugged in, powered by the S7LN PSU?
he can
I have a 1 board s-9 does 4200 gh uses 430 watts
old firmware set to freq 525