The Dee Howard/Alenia conversion put three Tays on, not two. I don't believe there was any conversion that put only two Tays on.
There was however one which put two JT8D-200s on (Valsan, later Goodrich).
The FedEx programme involved extensive acoustic engineering on the original engines.
Quite a good summary here:
http://www.macavsat.org/pdf_files/mi...kit_report.pdf