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@M_V
Some thing there aint right. Dura Ace 7400 hubs should be neither 7 speed (I don't think?) nor screw on.
I think they should be 8 speed which could be used with a 7 speed cassette if a spacer were fitted.
Obviously, if that's what you have then remove the 7 speed cassette, remove the spacer, fit an 8 speed cassette and Robert's your mother's brother.
However, if you have a 7 speed hub the freehub body will not be wide enough to fit an 8 speed cassette on. You could change the freehub to an 8 speed one but then you also have to respace the axle, if it's long enough, and re-dish the wheel slightly.
If you have a hub which takes a screw on freewheel then you could fit an 8 speed freewheel. They aren't common and they aren't exactly high quality but they do exist, probably only in mtb tooth counts though. No idea what would happen to the axle spacing.
"BSC screw on" could refer to a uniglide cassette rather than a screw on freewheel. Incase you are unaware, uniglide uses a screw-on small cog to hold the others on the freehub body rather than a lock ring. The final cog is female threaded, hyperglide lockrings are male threaded. For a while freehub bodies had both uniglide and hyperglide threadings and I think 7400 was out around this time. Not sure if you get 8 speed uniglide threaded freehubs or only 7speed though.
If you can post images of the hub and current cassette especially around the small end of cassette and axle area then it'll make diagnosis of what you need easier.