Thanks to this thread I have now fully converted my FC660C for the Heavy-6. I used the Niz sliders, JTK sliders for the mods and Axioms 3d printed stabs for the spacebar.
I accidentally didn’t use the smoothest printing material for the space stabs which was an issue I could mostly fix by filing the plungers down a bit. There is still some friction there but with a BKE heavy dome its fine for me under normal usage.
The JTK sliders are a bigger issue, the SA caps fit too loose or sit too low, specially the enter key feels like it can leap off the board if I try hard enough. I used o-rings in the caps to help them sit at the right height but I might try some thin plastic foil on the sliders to tighten them up a bit. They don’t feel or sound as nice as the Niz sliders.
I only received SA Spacecadet about two months ago at about the same time as I received the Heavy-6. It was the last SA set I ordered (fall 2018) since just like many others I had become a GMK fanboi. And I didn’t plan for it but boy, is this a great combination! Despite the 2u mod issues, together with BKE domes and Hasus FC660C alt controller I am very happy with the end result.