Question about tach drive?

[C6AE]

Hi all,
I have posted a bit about my '62 XLR basket case in the pushrod thread, but now have another dilemma
(There are at least a hundred of these dilemma's - hahaha!)

I have two tach drives (they fit where a magneto/dist would on the street engines, both are 2:1 ratio.

One (all aluminum) has a slotted shaft to accept the cable and the other which is early (factory made from a steel distributor drive) has a 5/32 female square drive similar to an automotive speedo drive
Which one should I use?
Are tach cables available to fit either type slot? The blade/slot driven end of this cable is not something I'm familiar with.

(I would have to blind broach the 5/32 square hole into an steel shaft, so obviously the slotted one is a simpler solution for me)

My assumption is the driven end of the cable looks like this (British) unit



thank you