Okay guys, I think I have it narrowed down for my issue. I think I have an ECM grounding issue. I've read other posts and found where guys say they go, problem is I don't see the wires.

Other than the main ground, the braided chassis to block and the tranny plus the deck lid I don't see anything.. I have a ground through the starter bolt, green and black wire.

Terminals D1 and A12 on the ECM look a little blackened, I have no shorts between any pins other than grounds to grounds. Where are these damn ECM grounds? I don't remember taking off any other grounds during the rebuild other than the mentioned ones...

The ECM is grounded to a bell housing to block bolt/stud above the starter.

ECM has Several "grounds" only 1 or 2 are real grounds. Others are return lines for ignition and O2 sensor that must be good.
Tan O2 "ground" bolted to block.
ECM ignition "ground" is connected to the ignition.
See my Cave, Ground "Myth" notes

for ends bolted or screwed to whatever... clean/fix and coat w/ silicone or brake grease.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Answered my own question after looking at a schematic and realizing I was being stupid and hastily hooked up the starter.. Guess what wires are right by the starter?? The grounds for the ECM and O2. So hopefully this helps someone in the future. If you did a rebuild on a 2.8 and your having ECM problems, pop down by the starter and see if you hooked up a green/black bundle and brown/black bundle on the starter lug.. ya know cause they fit perfect.

I took that section apart quickly when I removed the engine, pay attention to the harness in particular when doing a rebuild. Take notes, take pictures and go slow. Luckily I didn't fry the ECM, just melted a few wires, simple fix but a lot of stress. These are the most frustrating cars yet the most rewarding, don't know what it is but their like an addicting puzzle

There is an adage among mechanics....

"Know how to reassemble before disassembling."

It's very helpful to keep in mind.

Grounds are critical to proper operation. My 3800SC had developed a random misfire and was setting an 0300 code. I suspected that it was the coil pack or ignition module. On a hunch I removed the coils, module, cleaned the mounting surface. Then coated with conductive grease and ran a dedicated 16 ga ground wire to the mounting bracket. Started it back up, the problem was gone and the engine ran like new.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "