well its a bit hard to put the tach lead on a spark plug wire when you can't get to the wires. not sure how the testing is out there, but in pa we're not required to remove anything to get to the wires. there are 3 other ways to pick up the rpm signal...
a ground lead, very inconsistent, especially on well built cars.
a battery lead, same deal as above.
an "antenna" placed in the vincinity of the distributor.
if none of these 4 ways can work, we can run it "rpm unknown" and it just gets a one speed idle test.