Cougars and Mustangs are the same under the skin from the very front all the way back to the frontmost rear leaf spring eyebolt. This info taken directly from the Ford Chassis Manual. From there on back they're different. So, to answer your question, Mustangs and Cougars would use the same subframe connectors.
Of course, you could always make your own weld-ins from square tubing. If you look at mine in the Photo Gallery, I had even recessed them into the rear floorpan so they wouldn't hang down and look dorky like most of the others I've seen.
Now, as for the convertible issue, my buddy Terry is going to be adding the same sort of semi-inletted connector to his 73 Mustang conv't this summer. His car was put together from a real basket case, using the technology we had available at the time twenty years ago. We did replace the inner rockers (the real frame on a conv't) but the car still feels like it is wobbly in the middle. That's why Terry is going to add the frame connectors -- to see if it will rid the car of its jitters/creaks/driving-with-a-broken-leg feeling. He will be going the semi-inletted route since it will tie all of the floor pans, seat reinforcement and both subframes together into one solid integrated unit. Hell, he's got my vote. I can lift one whole side of AlphaCat by putting the floor jack under the middle of the newly-installed frame connector so that has to be adding a good degree of structural integrity in there! It looks like I'm doing NASCAR tire changes on it when I do that now, LOL!
