I've got the DR400, the steel and ally swingarms interchange. The cheap DR400 mod in the UK is to put a set of Armstrong rotax 4 stroke army bike shocks on. They just happen to be 16.5" Ohlins. The Harley 350 shocks are probably the same. The bike got 3rd in the 83 Dakar. Last of the twinshocks and the smallest in the top 19
http://www.dakardantan.com/forum/photos-paris-alger-dakar-1983-vt1006-30.html near the bottom of the page. I'd bore the yokes and shove in early 38s too, the stiffest Maico springs go in, that's what hold up my bike. Piston rings for the 400 and 500 are identical, as are some of the bolt on bits.
Tests at the time said the motor pulled better than the XR, it's probably tougher too.
The UK never got it. We got everyone else's discounted 400s for a couple more years, yet another recession!
In the USA the road going version of the 500 was designated the SP. As far as I can tell that model was sold as the euro DR. I doubt europe got the offroad version.