8 speed cassette on a 7 speed DA 7400 hub - possible?

The only issue with re-spacing is that the easy-to-find cassette spacers are not the same as those used in freewheels. The 7400 freewheel body is larger in size than a freehub body IIRC. For that reason, I'd maybe just get a spare shifter.

Ah yes, I was forgetting that we were talking about freewheels as opposed to cassettes.