I don't know if they interchange either but it would be strange for Ford to build different instruments for cars that are pretty much identical. It might be possible that the Cougar tach is a larger diameter but I don't know for sure. If they do they go right back on e-bay. Nothing lost.
All I know is that If it looks like a duck, walks like a duck, and quacks like a duck but someone tells my it's not a duck he'll need to prove to me it's a goose or I'll have to prove it to myself. Hell, I just bought a pair of Cougar interior quarter trim panels with armrests to see if they will fit in my car. I'm betting they do since, again, we're talking about basically identical cars built on the same assembly line with the same parts.
No guts no glory. Of course that's easy to say since if they don't fit I'll just clean them up and stick them back on e-bay, or I'll pull the arm rests off them and put them on my panels.
Cougar parts are still dirt cheap and we need to tap into that market before others do.