Hi Dave
I just finished assembly of my 550K1.
My experience - wobbling at low speeds are caused by tire, its pressure (too low) and head bearing.
Forks - I did complete disassembly and cleaning, then assembly back and new fork oil Motul 10W, 160cc each leg.
I assume my fork is also 100% stock, springs lengths are 460mm. Despite I tried, I was unable to find service limit for my springs... Seems fork on CB550K and also later CB550F are different than my K1. Either I'm blinnd or just stupid, I did not find it in service manual, nor in Haynes book. I did not find exploded view of my fork in Service manual too (I have 2 small springs at bottom that are not shown in manual but are in spare parts catalogues on Partzilla, CMS....)
Wobbling/Tire - I would look at tire (do you have new one?), pressure? Maybe was standing deflating for years on one place and now tire has changed shape that is causing wobbling...
I have old( >20years) Kenda, it was wobbling too when I first tried bike when I received it.
BUT then I have replaced steering head bearing by tappered one and now, with old Kenda no wobbling at all, head bearing is a MUST change on these old bikes. Even new standard bearing will do the job...
I ordered new Avon Roadrider's AM26..
Note: On my CB750 K7 I have Battlax 45 and these are performing very good.