I found an old "how to guide" on the kayjey's website which explains how to fit an AFR gauge to the ECU.
http://fiatcoupe.net/howto.php?ID=9&chapter=18#content2

One of my friend wired his Blitz SBC ID3 to the lambda wire, and now he gets the AFR. He told me that he has the good data.

Do you think it's a good idea to fit an AFR gauge without a large band ?

lamba wire will be fine

I think it is pointles without a large band.

plugging it into the ECU loom does nowt, the best way is to fit a separate WIDEBAND lambda, as the OEM is not.

I had a custom downpipe made with a welded boss fitted ( although any good shop can do this in-situ on an OEM downpipe ).

point, is as Ken says, you need a wideband, as otherwise you'll never see how badly / good the engine is being fuelled

I thought i could get valuable datas with that type of plugin.
So you advise me not to fit an AFR without a wideband lambda.

But what about the Blitz SBC-ID. I have 2 friends who swear me that they have good walues with it.

The narrow band lambda sensor will only give accurate readings around stoich. It can tell you if you are really rich or really lean but the resolution is very poor the further the fueling moves from stoich.

thanks buddies !