Do I really need a wideband to tune an LS?

For anything beyond small changes, yes.

• Narrowband sensors only read accurately right at stoichiometric (about 14.7:1 on pump gas). Richer or leaner than that, they're effectively useless for tuning (HP Academy).
• Wide-open throttle runs well rich of stoichiometric, so a narrowband can't tell you how rich the engine actually is under power enrichment.
• To calibrate across the whole operating range, you need a wideband. LSX Magazine says the same, and PCM of NC calls wideband feedback essential for cammed and forced-induction combinations.
• For remote HP Tuners tuning, PCM of NC wants wideband data brought in through the Pro interface or a CAN wideband, not a generic 0–5 V input.

The stock narrowbands still do their job in closed loop at cruise and idle. HP Academy's target there is fuel trims within about ±3%, with up to ±5% tolerated.

Want to do it yourself? See who tunes it, and how to learn.