I tried again today and when I failed, I removd the Pan & Valve Body. I put the VB on my work Bench, pushed the Plug holder in and while it looked like ir was in, I gave it another, harder shove and the Plug Holder went in with a satisfying 'click'. The screw turned, started and tightened properly. So it was me all along - I thought it was pushed home, but it was not fully seated. I put the VB back in but it started getting dark and my 71 yr old back & shoulders were aching, so I decided to finish it tomorrow...

Sorry gang, it was me all along, I was not properly seating the Insert correctly. Hopefully I will be able to get enough leverage tomorrow to push it in all the way & I'll finish this job! Sorry to have wasted your time, but I sincerely appreciate everyone helping me.
LTurn9