OK, found a diagram for the '92 Astro Van 4.3 H.O. A bit different from the one above, and more legible.
So, if you do a line-for-line comparison between the 2.8 and 4.3 HO, you'll find that there is wiring that's used only for the 2.8, and the HO motor has other wiring and components specific to that motor.
But, there are some things that have the same function and the wiring in many cases goes to the same terminal.
IMHO you're gonna have to use the 4.3 harness and ECM. Hopefully you got that out of the van.
But before you rip out the 2.8 harness, I would look at every external input/output from the dash area, fuse box, and relay box. There are a number of cross-over items, and if you can retain the original function, all you'd need to do is pull that wire from the 4.3 ECM connector plug, and insert the appropriate wire from the 2.8.
Otherwise, you're gonna be pulling wires out of the dash & fusebox & re-running those leads. No sense in doing that if they can be re-purposed to the 4.3 ECM. I did a quick comparison and it looks like all the fused (hot at all times) or fused (switched) power can be re-used.
The 2.8 uses a "chopper" in the speedometer head to feed the speed buffer (VSS), whereas it looks like the 4.3 uses a VSS sending unit at the speedo gear in the tranny. I'd bet that it's the same signal and you could take the feed from speedo buffer and stick that wire right in the VSS signal position on the 4.3 connector plug. The VSS output goes to the same pin on either ECM.
Now, stuff on the engine like temperature senders, MAP, TPS, ect, will be picked-up by the 4.3 harness. And we don't care about the 2.8 wiring that corresponds to that stuff, since it's from the engine to ECM, not the hard-to-reach stuff.
BTW, wonder where the Astro Van mounts its ECM, in the glovebox? That might require some re-routing or extension of the 2.8 ECM switched Power/fused power wiring. Or maybe there would be enough length in the 4.3 harness to make it to the center-console location of the 2.8 ECM.
The 4.3 doesn't use a main relay like the 2.8, but it looks like you could retain that (it does serve other functions for HVAC, as I recall, that won't work if you disable it). You can re-route the power coming out of the main relay to do the same function for the 4.3.
The 4.3 doesn't use a backup fuel pump relay. From looking at the diagram, the backup relay pretty much becomes useless if everything is hooked-up as per the 4.3 wiring.
The oil pressure switch would have to be reconnected inline with power to the main fuel pump relay 'cause it's inline with the backup relay and that wouldn't work. But maybe the 4.3 harness will pick that up anyway??
The 4.3 O2 sensor has a heated element so you'll need to tap the heater line into switched 12V power.
Stuff like that! Just a cursory review but I'd think it might be too much of a pain to try and use the 2.8 harness and add wires.
The 4.3 has an Linear EGR Control, Variable Tuning Control (whatever that does!), and no external Electronic Spark Control.
It certainly looks do-able, just need to "Nuke it Out" before ripping things apart.
IMHO saving wiring out of the 2.8 harness that have the same or similar function for the 4.3 harness, would be the way to go.
Painless Wiring did sell a stand-alone harness for the early ('92-'95) Vortec 4.3, but I couldn't find the part # (60210) on the site.
Found it here for $612.06:https://autoplicity.com/368976-painless ... mNOFITMENT
$486.99: http://www.streetperformance.com/part/p ... 60210.html
Here's the installation manual for that, lots of good info in there:http://www.painlessperformance.com/Manuals/60210.pdf
Of course you won't need that if you have the Astro Van harness.