Success!
Tried the 1/4" Craftsman ratchet & 12 mm 6 pt shallow well from my 'on truck' tool set that Bill had recommended. Laying across the engine and my son making sure the socket stayed on the nut. Pried it apart and stuck the blocker plate in. That was a bit tricky. Had to spend quite a bit of time tap, tap, tapping with a small brass mallet but got 'er done and tightened back up.
No codes on the test ride so the finger stick install from earlier must have been correct.