You'll want to check if you have the speed sensitivity solenoid that came on later model HDJ81s. If so you'd have to research the differences between that and a standard box. The lines may be different and the steering shaft is shorter. I wrote a little about it in my build thread but haven't personally changed it out yet.
SNLC confirmed that he had changed them out - it's not hard but not plug and play.
I bought a second HDJ81 steering box out of Japan from autoparts.beforward.jp and will probably rebuild it.
Cruiser Outfitters also carries steering boxes - they just posted about it yesterday: