Hi Eric Because you are going to be using new rings that did not come matched to your pistons these measurements are critical, but most critical of all is measuring back clearance of the rings. The Jaguar manual I’ve got (for an E Type) doesn’t mention this, I suspect because they thought you’d use factory purchased rings?
Take a look at this site https://www.hastingspistonrings.com/tech-tips-faqs/ring-groove-depths-of-piston-rings and it will explain how to do it. Having said that I’m going to tell you I know more than Hasting about doing this - lol.
They show the ring inserted on the thrust side of the piston. Not on your pistons - sorry. Measure it there but more critically measure it over the pin bosses - that is 90 degrees from where you see it in their diagram. They also say it’s ok if the ring, once inserted is flush with the surface of the piston. Maybe but I’d be a lot happier if the ring went further into the groove - say by .005 to .007 - that is more back space. This additional space is needed to ensure good compression by permitting combustion gases to get behind the ring and push it into the wall. Why all the concern of checking over the bosses. Most piston are cam ground - that is, when cold they are a very slight oval, the narrow part of the oval being over the pin bosses, where most of the aluminum is. When the piston heats up the pin boss area expands more than the thrust sides, and the piston becomes a circle. Most manufactures however cut the ring grooves on a lathe so they are circular when cold - when the piston heats up and becomes circular the grooves become slightly oval, the widest part being over the bosses, pushing the ring towards the wall of the bore. It there is hard contact between the back of the groove and the ring and the wall you won’t be a happy guy! That’s why this area is the most important to check.
As to how to measure - if you have a reasonable set of calipers you can measure both the inside of the groove and the thickness of the ring.