Kria will probably not be usable since you can’t access most of the parameters pages. Meadowphysics and Earthsea may work okay but of course half the control surface will be inaccessible. I know some users use 4-step 128s with Ansible apps successfully.
I have never used an Arc and am not too familiar with the Ansible apps for it, but this may work out a little better just because I think Arc rings map more one-to-one with Ansible’s output channels.
Certainly! Source code is here. Modifying Kria to work with a 64 is probably not possible, but changing Meadowphysics and Earthsea to only use 8 columns seems feasible.