#37846-99a came on all 1999 HD EFI models and was first introduced in 1999 as per the "-" designator. It was used on 2000 and 2001 EFI models but no longer used in 2002 when all models got the 3.15 gearing.
To go to 3.37 gearing you also need P/N 40269-85A compensator nut which was used on all HD models through 1993 and then only on the later EFI models through 2001.
To go to 3.37 gearing you also need P/N 40269-85A compensator nut which was used on all HD models through 1993 and then only on the later EFI models through 2001.