Excellent idea----comparison photos showing points to be aware of. No doubt, undercarriage should be given priority in consideration of the condition of most of these old tractors and in most situations may be the most expensive repair item experienced in a restoration/rebuild.
One point that needs to be considered when looking at the distance of the front idler to the front of the track frame is verifying the proper number of links in the chain. A link may have been taken out to shorten the chain---and allow you to continue to run for a short time, but will result in the chain climbing the rear sprockets due to the elongated pitch (internal wear in the chain).
Therefore the importance of measuring the pitch and counting links (as well as comparing other dimensions).
I feel confident that Old Magnet will put us together a very helpful reference guide that some of the other "old pros" can add to.
Excellent "candidate post" for Archives.
Avon, Ms 38723
D2 5U and other scrap iron