I had almost no problems with the sixth plug.

It's much easier to get to if you unscrew and move aside the nearby brackets for the brake booster vacuum hose and throttle/cruise cables.

Also slide off the booster vacuum hose connection at the manifold.

I found a workable combination of tools to be plug socket / 3" extension / universal joint / 6" extension / rachet as needed.

You can look right down on the plug with mirror (situated behind the brake lines) to determine its orientation.