You can get the Rev8 WD SCSI chip from Centsible for 26.00 USD (Ignore the picture).
http://www.centsible.com/Softhut claim they have them but I think they just never update their website.
Amiga Shop say they have both, but I have never ordered from here and they dont have 2006 on their website!
What is more important is a Buster Rev11 and kickstart 3.1 (and possibly an INT2 modification.)
If you need your A3640 upgrading to 3.2 let me know.
Good luck finding them. Let us know if you do.