I am almost done installing my Boston Bob engine.  Last night, I installed
the large aluminum heat shield (behind the muffler/cat) that bolts to the
motor mounts and has 4 small bolts across the back.  (This is a '85 if that
matters.)  These 6mm bolts screw into some kind of blind threaded insert.
Two of them just spin.  Evidently, the original bolts rusted fast and when
they were removed, the insert broke loose.  I wondered why those bolts were
missing!   One in the center has a bolt which is part way out and stuck.
Any solutions to this before I go any further?  I don't think you can drill
them out--they will just spin.   Is there any access from above?  It seems
to be between body panels.  It can work without.  I have two that hold,
plus the front side is bolted on to the engine mounts very securely.

                                                                  Jay