You wouldn't want to mix parts from the two...they are completely different platforms. The Cougar is more closely related to the '71-'73 Mustang. Those are unit body cars with leaf spring rear suspension, while the '72-'76 Torinos had a full frame with coil springs front and rear.
All of the Toronado and front-drive Eldorado kits include Oldsmobile engines. '65-up Cadillac V8 engines had the distributor at the front, among other differences. Jo-Han never tooled a decent late Cadillac mill (nor did anyone else, for that matter). The Oldsmobile engine's valve covers had no lettering or script.
Model Car Science ran a two or three part article on building a 1/25 scale Batmobile back in the day. I haven't got the issues (don't collect MCS) but if I remember right, the builder started with a '65 or '66 fullsize GM car kit, and advised readers to use one too. I think he stated that it really didn't matter which one you used as a starting point (Olds, Buick, or Pontiac) because so much of it was going to be reworked anyway. The conversion involved a lot of cutting, sheet plastic, and putty, but the finished model looked pretty decent. Some kid probably saw one of those articles, grabbed the closest thing that was available, and got to it. And probably had a lot of fun doing it.
The Round 2 MPC Cosmic Charger dragster includes a newly-tooled Potvin blower drive setup; that one should work, and taking those parts out of that kit won't leave it unbuildable. This won't be of any help unless you could track down the parts...AMT did include a similar setup for the Chrysler display engine in their '61-'62 Buick Special wagons. The same display engine and trailer were then used in the '63 Nova wagon. The Nova wagon was later butchered into the Boss Nova (using the Chrysler engine to power the car). Unfortunately the Potvin setup wasn't included in the Boss Nova because in that kit, the engine is right behind the driver, and that setup wouldn't fit. So you'd be looking for fifty-year-old parts. It wouldn't be tough to scratchbuild/kitbash a convincing setup anyway.
AMT and Revell/Monogram both overproduced on the kits, in addition to that there was a proliferation of diecasts alongside those. Then the bottom dropped out. There are exceptions, but once they went to the downsized cars in the early Eighties, you don't see a lot of nostalgia builds on these (certainly not as many as pre-1980 cars). I know a guy who was getting sealed Nineties kits for $1 apiece up until a couple of years ago. He'd resell them for $2-$3 apiece at a local flea market. I myself decided to pick up a couple of the early Monogram kits a couple of years ago. I got one of the first four kits (the red Buick Regal), sealed box, for something like $3 (not from the guy referenced above). That said, if you are into the subject matter, that's good for you because you can stock up for cheap. If you don't care for NASCAR but do kitbash and build street machines, the chassis parts can come in handy. The truck kits have a slightly longer wheelbase; those chassis can be fitted to earlier bodies instead of stretching the car chassis (not that that's tough to do). The engines in most of the kits I have seen are pretty decent, too.
The first AMT Vanishing Point issue (a few years ago) had the twin-scoop hood, the correct style wheels for the VP car (don't remember what they were called), and a front bumper with no molded-in guards. I'd imagine those parts will be in the new issue also.
Last week, I got a call from my mom's care facility telling me they had sent a registered letter, containing a form I needed to sign and return. That letter was in the mail already when they called. We went over the information on the form, and I disputed some of it. So they had to send another registered letter with a corrected form. I got a little green postcard in Monday's mail, and went to the post office on Tuesday and signed for the second (corrected) form. The first (incorrect) form, sent at least a couple of days before the second one, arrived today.