Another thought

I got this old HP counter, and it is a good candidate for a conversion to a clock. 
Since it is not a true frequency counter, just a pulse counter, it has limited uses these days...

I'm pretty sure it has the exact same driver ICs as the 10 nixie display board.

So all I would need to do is cut the traces for the ABCD inputs to the nixie driver ICs and substitute my own circuit.