Hmm. You're going exactly the other way from me on a recent project I've been working on.
The stock XJ/MJ sender (the 3 pin one) is 8 pulses per revolution of the sender shaft, which with a proper speedo gear (supposed to aim for 1000 revs per mile) means 8000 revs per mile. I don't know what the chevy ECU wants but I'm surprised a stock chevy sender won't fit into the case, aren't they almost all the same on 231J/231D/231C?
If the Chevy ECU wants a coil+magnet style pickup you might try a 91-?? (I want to say 93, but am unsure) Jeep VSS sender, which is a 2 wire coil + mag pickup. I don't know how pulses per rev compare with the early HO sender vs the chevy sender unfortunately.
In my case, it's a 99XJ NP231 in an 87 YJ with an 86 CJ dashboard and mechanical speedo in it that I'm building up for a friend. So I threw in a mechanical speedo sender from a 90-earlier XJ/MJ and connected the mechanical speedo cable to it. Everything works, except it bogs and tries to stall when you clutch in to downshift at speed because it thinks the vehicle isn't moving. Since there's already a speedo gear drive in the case there's no space for another to make the ECU happy... I am investigating using the 87 YJ cruise control speed sensor halfway up the speedo drive cable, unsure if it's a reed switch or mag+coil pickup nor how many pulses/rev it outputs at this point but a little playing with an oscilloscope and a DMM should straighten it out fairly quick.
Hopefully this helps or gives you some ideas.
