My #3 connecting rod was making a loud rattling noise mostly when decelerating. In adjusting the cap clearance I went the Plastigage route and struggled for 3+ hours. A combination of old eyes, hands and a belly in the way made the project more difficult than it should have been. In desperation I went the old tried and true way of using a 1" piece of newspaper to set the proper clearance and it worked like a charm. #3 is nice and quiet now which allows me to hear a faint rattle from one or more of the other rods. Forget Plastigage! I'll be using newspaper for future adjustments.
John: You could use the Ford method as described in Par 386, page 99 and shown in Fig 237, page101 in the Service book. It worked then and still works. Dan