Yes, maybe someone else can step in here, but i believe there were some differences in c4's throughout the years that could make this an issue. all i know is that when i changed out my c4 in my 65, i could not use the nss on the new tranny. it wouldnt fit. i purchased a 70+ c4 with the 26 spline input shaft. i dont know which one you have. and im not sure if my issue was just a quick oversight or mistake on my part or whatever. i believe it had something to do with the fact i was using an aftermarket
hurst shifter instead of the factory one, its hard to remember since it was a while ago. but in my case, the new shifter came with the switches built into the shifter instead of the on the tranny type of deal. i would buy one for a 66 and see if it works. it should, but this is where my experience ends. like i said, i had different circumstances