I’m new on this particular forum but hope posting this helps you. I changed out the ECM on my former medical transport 1993 E-350 EB Hightop last Fall.
Mine is a $700 project camper van equipped with a 5.8 with 131,000 miles. It has been dormant since about 2004. It spent most of its career in AZ and later came to TX. The only rust in is in the battery tray area(s). I pulled my ECM to check the board as preventative maintenance.
I was trained to be a technician by UTI in the 1980’s but only worked as one for a few years. I still work on vehicles very frequently.
Long extension group to make accessing the ECM screw easier.
Not the best photos –after removing the plastic/rubber cover - 2 nuts retain it - I pulled straight back on the ECM connector.
You can see the two studs on either side of the ECM - these retain the cover. Hopefully you didn't miss those fasteners.
There is a plastic/rubber shield that goes over the connector. Maybe yours is adhering to the connector or the firewall. Trying using a small hook tool around its edges to loosen it. Just use your hand to remove it – not the hook if you can avoid it.
My original ECM showed evidence of the very common capacitor leakage issue (brownish stains).
I acquired a “refurbished” ECM. Cover installed over it. Nuts are being installed.
Hope this helps.