FWIW to anyone else who may be courious, the answer is yes it will. I contacted R&H Motors in Owings Mills MD and Eric was able to confirm that the old and new part #s for the 420 SEL air bag and 300TDT air bag are the same.
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)