65 or 66 will work, 66 supposed to be made of heavier gauge material. Canadian poncho will work as well just have to change the rear most cross member it's different. I have a 65 CS frame under my SS no issues other than changing out the pontiac crossmember for a chevy 1.
67/8 are "supposed to fit" can't say for sure never done that. Might be differences with the front frame horn. Would be nice to know if they did as you get the better steering geometry with them.
Try classic auto parts in Langley. When I was there last spring with my brother in-law gathering parts for his 65 valiant I did see a mid sixties impala buried in a blackberry Bush. I was just there this week but never ventured into the yard, it was partially submerged in water, bring gum boots and a machete if your going to look. He still has a few buried treasures out there.