The ground from the side of the block to the battery negative is important, but for a whole lot more than just the sensor reference which gets there through the sensor body to head, head to block through head bolts. If you had high resistance there, it would show up first trying to crank the motor, but worth checking.

The firewall to cam cover ground is not needed except to reduce radio interference.

Measure the voltage at the ECU connector as the motor warms up, and match it to this chart rather than trusting your ohmmeter skills.




--
Art Benstein near Baltimore

"If you don't know where you are going, you will wind up somewhere else." -- Yogi Berra