Doing a 5.0 Conversion Question?

I beleive you are correct. A 91-93 engine harness is needed to mate up to a 91-93 dash harness like OEM.

The plug connectors between the two changed several times. I beleive 87-89 are the same, 1990 is unique and 91-93 is the same.

I am sure someone who knows for sure will chime in