Final addendum to my build thread. My 4.3 was a frankenengine, with an early block, later heads and an Astrovan cam. That meant there was no stock fuel/eprom map that worked well for it. That's not a huge loss; the stock mappings for the 4.3s don't seem to be all that great, and largely account for the V6's reputation for having "4 cylinder power and V8 fuel economy". Fortunately, tuning the early 90s GM engines used to be a big thing, and the know-how is still out there along with the tools needed. I've been scouring forum posts and tutorials, trying to learn 30 year old fuel delivery technology.
You program some settings, burn a eprom chip, and go test it out with the laptop in the passenger seat collecting data. First you get your fuel map so the O2 sensor reports good fuel burn under all conditions. Then you start carefully tweaking the spark advance. There's stuff like "accelerator enrichment", which is the equivalent of a carb's accelerator pump, and dozens of other settings. It's been a long, sometimes frustrating journey. At one point my rig was throwing an error code for the knock sensor. It turned out my spark advance settings were so conservative that when the computer tried to run a routine to test for knock, it couldn't make the rig knock even when it bumped the spark advance considerably under full load, and so it thought the knock sensor wasn't working and put the rig in limp mode to save the engine.
But...
When it finally all works, it's a glorious thing. I've got 3:55 gears, limited slip and 31 inch Goodyear Wrangler tires, and I can chirp the rears if I stomp it off the line. Yesterday I was doing a final datalogging run, climbing a 6.5% grade, and walked past a fancy Dodge pickup that was doing everything it could to keep me from passing. Heh. That was fun.