It did the pull up to a stop sign and die thing with me
The only other option (other than the rather sketchy Dakota Digital adapter) is to pop the plug out of the throttle body, and set a "minimum" idle speed. It kind of over-rides the ECM and ISC.
Here's the long story, because I don't have anything better to do...
What happens is that, if you're coasting with your foot off the gas, in gear, the ECM is trying to slow down the engine to idle speed. (Remember... it sees NO meaningful speed input.)
It tells the ISC to reduce engine speed. Of course, since it's a manual, and you're coasting in gear, the engine does not slow down. The ECM finally closes the throttle completely, trying to get the engine to idle down.
When you finally push in the clutch, the engine stalls because the throttle is completely closed, at the behest of the ECM/ISC. Hence my comment about setting a "minimum" throttle opening with the throttle body screw.
...
Assuming that the 4.9 originally had a 4T60, it's PCM will want to see the same kind of VSS pulses, and I'm sure they are different from what a Fiero-type VSS produces.
Exactly right. Different in wave shape and frequency.