tp2177, I don't have any opinions for you on the archer. However, I have run the D196 (homie, poulans) mount bars on the D176 mount that 590 uses with the cheap S-clip or make my own. The 196 mount bar would need no other mod. (the 009 mount common with Husqvarna could also possibly be used, but never actually used them this way as didn't have any to try...I've seen others run 196 mounts on husqvarnas though).
Another option could be swapping the bar studs out from the 590 to bar studs from a 196 mount saw. This would be if they are threaded in the case on the 590. On olympyks, old jonsereds, homelites, some poulans having 196 mount size studs, it is easy to make an s-clip and mount stihl bars to them with no mods. I have several 025 mount bars that run this way on some old homelites, olympyks, and macs.
You are thinking correct on going from the 176 mount to 025 mount that would need a spacer.
This weekend I can send you some pictures if interested. I may have sold all my s-clips to go from 176 to 196, and if so should make some more. On my personal saws at present I have several clips or spacers to run the 025 mount bars just cause they are so prevalent and always can find cheap deals in that mount.