If I remember correctly the 400 uses a seal with metal reinforcement inside it, and they do work, but only if your speedo gear shaft isn't grooved.
I just put 2 standard seals stacked for grooved shafts, and a slightly larger O ring on the gear housing with just a light film of RTV on the case and then tap the gear housing in.
If you don't have the retainer for the inner seal, a coil from an appropriate size spring can be cut and used for that.
You'll want to make sure the ends of the cuts are smooth enough to not scratch the aluminum housing.
