Hi Old Fella,
My memory was failing when I posted on 17th. You are correct that the rudder boost was selected on most of the time, but Boeing published VMCA figures for Boost Off (about 60 kts faster). I found a copy here:
http://www.vpiaheritage.com/707limitations.pdf
on page 10.
I seem to remember we would turn the boost off in the cruise to dampen the yaw. Happy days!