difference between narrow and wide band A/F Gauges

The "narrowband" is for OEM applications, and is designed to read from about 13.5 to 15.5 to 1 accurately and reliably. The "wideband" will read from 11.5 or so accurately.

With most any carbureted setup, you are way on the rich side of ideal, and all those hydrocarbons will kill an O2 sensor. (Notice I didn't say reliably when talking about the wideband above)

Buy the wide band, but don't use it until you are reasonably sure of your tune... and then take it out until you need to do more tuning.

And syleng is right, even most of the $250 ones are "cheap." Do your research before buying.