From what I’ve been able to glean, the number 20 Gulf 917K was represented by three different cars in the film. Firstly the real JWA car that was actually raced at Le Mans 1970 by Sieffert and Redman. Second a more "normal" 917K that the filmmakers purchased from Porsche and painted up to represent the Sieffert/Redman car. And finally a Lola T-70 with fake 917K body work for the crash scene.
I intend to build the "movie car" that is seen in the bulk of the film with McQueen actually driving. There are several small differences between the two, like McQueen's car has the normal narrow rear wheel arches and slightly bolder numbering. Also the Gulf stickers tend to move around during the course of the film!
I bought my HRM transkit from Kevin at Strada Sports - highly recommend his services.
As for a progress thread, I’ve barely begun so there isn't any yet. Now you are blazing the trail it'll save me the effort.
EDIT: removed all reference to chassis numbers because a quick check found nobody seems to agree which was which!
