Yes, the left side of the dash opening had to be modified as well, the rallye dash cluster is a little wider on the left side as well. I notched and folded the lip back a little on the left to make it fit. Pretty much every single inch of the dash frame opening has to be modified to fit the rally cluster.
My heat controls are the earlier ones, I think they're '68? The dash that has a flasher switch. Anyway, my mode controls are not hooked up. I set the door to "defrost" and zip tied it there. The heat slider works as normal, it's probably hooked up with the later cable but I honestly don't recall.