Of course, the other one to do is the T-bird front end, but I've seen it and I'm not big on it. I think it just doesn't go together with the Ranchero body, whereas I think the Cougar one does.
Im sure i saw a pic of one once on the interweb, looked ok, but not sure about headlight doors on a Ranchero.....oh wait! scrub that, just remembered how cool they look on early 70's Chero's ...im still voting for the Cougar front end, i like the twin rectangular lights side by side, gives it a wider look.
A lot of people with the '66 Rancheros put the '67 fenders, hood, etc on because they are more desirable for sale. The '66 and '67 are the same sheet metal except for the front clip so it is an easy swap to make. Laters, Jeff