I'm not sure on the accuracy of a narrow band A/F guage to be honest.

A lot of people say there not worth bothering with.
And others say they can produce just as accurate results as widebands.

Stu@MDevelopmens uses the narrow band in some cossie tuning and finds the results to be excellent i believe.

You will need to by a lambda sensor as well as the guage. and have it welded into the exhaust.

Matt.