Clocks

[Katman]

I'm intending on running a 400 kat with 3 spoke Impulse wheels. As the speedo is for an 18" wheel, would I be best using 250 clox as these run the same size tryes as the Impulse?
Would the drive spigot thing from the front wheel from a 250 fit the 400 (17") & would that alter the speed?
What with the revvy nature of these bikes & the crafty plod that we have round here, I'll need to know my speedo is pretty accurate
The rev counter goes to 17k tho I guess that would show actual revs?

Does this make sense??