Gutted manifolds or not the A/F gauge still works the same way, so long as you tapped the upstream sensor - which by your color description of the wire - you did.
As for running an A/F guage, I suggest you send your gauge to gadgetseller.com and have him modify it so that it's resolution is narrower and thus more useful for you to diagnose your true A/F ratio. Granted, the O2 sensor is not wideband, but nonetheless, at least you will be one step close to seeing what your A/F ratio turly is.
