G'day Berger.
Clear as mud
. I don't think drags has got it quite rite with the 68-3287 springs, these are for late A65 , 3 spring clutches. The 68 number you mentioned for the early drum is for the early 4 spring A65's ( they mixed that up as well)
According to there list and another I've looked at & as you have found, you need the 42-3273 springs & 42-3197 cups. ( for 5 plates )
I don't know the dimensions though but they do mention the 42-3273 spring can be used on the small unit Triumphs with the 3 spring clutch ( lighter than the 750's).
How many clutch plates can you stack onto your clutch center ? ( has it still got the lip) you should be able to fit 6x plain & 5x bonded . ( there should be enough room to fit 1 more plain plate so as when the pressure plate is lifted the last plate can't fall off).
I've only ever seen 43t sprockets BSA number 42-3266 & Triumph equivilent. They work with both center types.
You need to have a 5/6 plate set up, Ignore all the guff about 4 plates ( early pre unit triumph? C15/B40? )
With the spring cups fitted in the pressure plate do they bottom out on the backing plate?
If all 4 cups are the same length ( & clear the backing plate)
All 4 springs are the same length ( don't coil bind) You should get a workable clutch if all the other bits are up to scratch. ( any grooves in the drum from the drive plates ? )
If only we could just pop down to our nearest BSA dealer & buy the correct genuine parts instead of ( it might fit pattern parts/rubbish )