I know for a fact Mr.Heaterbox does rebuild and calibrate clusters to KM/H.
Whether he can do a 66 Chrysler you have to contact them.
The family is good to deal with and their work is too shelf.
Plus they are Canadian.
Tell em Leaburn sent ya!!
Welcome - Premium Dash Decals by Mr.Heaterbox
I had two 1966 Chrysler clusters rebuilt by Mr.Heaterbox. I've only installed one in a car so far, but they seem to have done a good job.
However, they took a very long time, so either send them a spare one to overhaul or remove it when you're parking the car for the winter and ship it to them ASAP so you'll hopefully have it back for spring. If they're like other services for car restoration, they're probably swamped with work because people have been at home working on their cars more because of Covid.
More importantly, the odometer mechanism is apparently unique vs other Mopars, and they don't have replacement parts to overhaul that. I had to send them all of my spare clusters (I have 4 or 5 extras) which they tore apart and took the best parts. So unless you have some spares to send along, or they've stocked-up on parts for 66 Chryslers since my dealings with them, you may have a problem getting your cluster rebuilt. Ask about that when you talk to them.
As for the MPH->KPH thing, you guys can't convert in your head on the fly? 30->50, 40->65, 50->80, 60->100 70->110, 80->120. Those are slight approximations, but close enough as long as the speedo itself is accurate.
Just FYI, most speedometers are designed so that typical highway speed occurs when the needle is pointing straight up. That way you only need to glance down at the speedo to know if you're speeding. You don't need to know the exact number and you take your eyes off the road for a shorter time. In a '66 Chrysler speedo the middle number is 60 MPH, which I would contend is just right.
Remember that brief period 1979-82 when speedometers were restricted to 85 MPH? The speed when the needle was straight-up was about 45 MPH. When people looked down at their gauges and saw the needle way over to the right, they were supposed to think they were speeding and slow down. The effect probably only worked briefly until the owner got used to it, but that's basically what you're talking about doing with your cluster. In Ontario, most people drive 110-120kph on the major highways, so your needle would be pegged a lot of the time here.
If you want to try your idea, I like the suggestion to change the speedometer gear and/or get an adapter to speed up the speedometer drive. That would be the fastest to implement and lowest cost, and would be easily reversible if you change your mind. You're talking about increasing it by about 60%, so you may need both a different gear and an adapter to get there.