I had a similar situation with my `87 El Camino I bought in 1993. There was a thin amber-tinted coating over the entire engine, inner fenders and core support. When I decided to get serious about removal, I tried every solvent I could think of except for lacquer thinner, since the paint from that era was all lacquer. What worked best was denatured alcohol, which led me to believe the coating was shellac-based. It took an absolute ton of effort, but eventually I got it all clean.
Bill