Have no direct experience with LM-1's, but a couple of local guys use them and swear by them. The key for us Cobra lovers is to have an O2 bung welded into the collector rigth before the muffler, for the LM-1 to read as accurately as possible.
There's a local carb tuner (Henry Olsen) who not only uses an LM-1, but also a 5 gas analyzer to do carb/ignition tuning. He tuned mine, and it runs like a champ!
Here's a link to an article he wrote on using a 5 gas analyzer to tune a Roush 427R -
http://www.automotiveu.com/tuningperformance.htm The key is to tune both iginition and carb together.
Regards,
Randy R...