I've used MSD's cam sync. They dont advertise them for Cs anymore but according to them the only difference between their C and W was the gear they installed. I dont recall the shaft diameter but C gears are available in any case. 351c distributor configuration is shared with 429/460 so any advertised for those should work as well.
For comparison, here's a Mallory 351C distributor next to the MSD 351W cam sync. As always, you'll need to check signal compatability with what you are using.
Kelly
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