Is this a stock '73 k3 Clutch basket?
That appears to be a K3-K6 clutch basket. The earlier ones that that had a steel collar added on the inner hub, but you'd have to split the cases to see if that was swapped in. The clutch cover and hub MUST come from a K3 or K4 engine (or possibly a late K2, after about 11/72 production) to get the spacing right for the clutch lifter, and to clear the ribs on the hub (which is missing at the moment?). Otherwise the clutch cover and ribs will scrape each other.
You will need a 7-plate (friction plate) clutch, with 6 steel plates. Despite what your friend told you, the OEM Honda clutch was good for 90+ HP, with nothing more than stiffer springs (like Barnett springs). The Barnett clutches don't usually hold that much power down, as they use 1 less plate overall (in my experience). I have replaced many Barnett clutches with Honda's own, and it worked better.
Honda is still selling many of these parts: I just bought some OEM replacements for broken ones I am rebuilding now, from Service Honda. I think they must still fit current bikes. You might wish to use Barnett springs on Honda plates, and make SURE the plates have square-cut corks on them. Many dealers are confused, are sending out the slanted-cork plates because of a snafu in the Honda parts codes since 2005. Those slanted-cork plates are only used in CB500 and as the top plate on the post-1976 CB750 bikes.
