On the octane, 104 equivalent research octane would be about 99 or 100 octane with the (R+M)/2 method. So that's about the same as 100 octane for a US reference.

Not a SB expert but I think 11.6 CR will be close with the torque cam... with the cool T stat and cool air probably OK.

I think you would be completely safe with 1/2 or 3/4 point less. Many/most KB pistons can take quite a bit of a cut. You could cut a step dish to drop CR (and would then need to re-balance) or just put it together and give it a try.

Personally I would not cut the head... keep it matched. No big harm if there is a bit less compression on one side.