the only conversions i would do, even today, include:

MR2 taillights (from the sw20 chassis, the updated ones) on DC integras.
Prelude Tailights 97-01 on 90-93 integra coupes.
04-05 subaru wrx front end on 06 subaru wrx (i like the 06 too, but the 04-05 is better)
s15 front end on s13 hatchback (180sx kouki)

other than those, nothing else really.