[Blue Bowtie]

I doubt that the starter mounting or grounding had anything to do with the transmission problem. The '83 used either a TH200-4R or a TH700-R4 depending upon the engine installed. If your trans has a square pan it is a 700-R4 (first year for that trans in a Caprice), and an elongated/tapered pan would be a 200-4R.

Neither of these use the ECM to shift the transmission, but only to control the converter lockup, and some of those early ones still used a vacuum switch to lockup the converter instead. In either case, it isn't the ECM. It would be good to verify the oil level (ruinning.hot) and possibly connect a test pressure gauge to read line pressure in various ranges.