Many thanks for all the assistance...and here we go with another PITA.
Just completed a coolant flush and partial hose replacement.  The plastic
thermostat housing is junk.  All four cover bolts are rust and the bottom
fitting spins; the cover can not be removed.  Next the bleed valve is stuck
shut.  So I buy a new unit from Ken W.  Low and behold the two Allan bolt
heads do not take the 6mm wrench, even when the recess is cleaned before
hand.
What suggestions do you folks have?
One idea is to remove the front exhaust pipes, disconnect the 278 hoses,
carefully cut away the existing thermo house so vice grips can get on the
bolt head.  A good overnight soaking in PB blaster is in order to.
Does anyone know the dia/length of the two Allen bolts required
A search of the archives indicates many despise this poor bit of
engineering, although my situation is not addressed.
Thanks for reading this and responding.

Chris Hawkins
Lost Nation, NH
91 Westy "Arkle"