Rear lap belt mount

["Ginger D.&Peter P." <gingerd@teleport.com>]

I am attempting to install rear lap belts in a '66 Square, and I can't
find the outer mounting location.  There is a mount for shoulder in the
window pillar, and two mounts under the seat at the center (about 6
inches apart).  The only bolts/holes I can find to the outside under the
seat are tucked under a steel sheet, and I think are supposed to hold
the torsion beam onto the pan?   Anyway, the hole through this steel
sheet is too small to fit the belts metal end onto the bolt anyway. 
Anyone able to point me in the right direction?

Peter Parker
Portland, Oregon