If memory serves me correctly, for the RPMs, I think it's wire #10 off the blue connector, should be white.
It's somewhere here on LS1Tech, I did a search for it years ago and found all the information, and confirmed it in my haynes manual.
Let me go pull a search to see if I can find it for sure for ya. Edit:
Yeah, it is the white wire, but since yer a 98 it's supposedly pin 35 on the other connector.
Remember the pcm outputs a 4 cyl 4 stroke signal even though we have V8s.
Perhaps this thread of mine will help you out: https://ls1tech.com/forums/nitrous-oxide/328586-help-bmn-nitro-controller.html