A few more things I would want to look into, in addition to the track width mentioned above.:

Being able to somewhat match the brake systems size front to rear would be a consideration. The Charger is a much larger car (there for assumed as heavier), it may have much larger brakes then the Dakota has, where the Cougar/Thunderbird, being older may have smaller brakes more like those on the Dakota.

Then the future availability of component parts is the next concern. With the Ford stuff being 90s stuff, will replacement parts be available in 10 years? The Charger is still being built, we know those parts will be around for another 10-20 years.

The wheel bolt pattern could also be a concern. I am assuming you have the 5 bolt Dakota front hubs. They have an American bolt pattern. I don't know if the Ford of that age is still American or metric, but I know the Charger is Metric. The bolt patterns are close, but not a direct match. I'm nor sure I would want to run the wheels that has the same bolt pattern on both the American and the metric bolt patterns.

As far as mounting the components, as long as the track with is close, you may end up building a rear frame section to support the independent suspension, but once past fabricating the entire system, it wouldn't be much different then splicing the front clip. Track width can be adjusted with an independent rear suspension by shortening or lengthening the outer drive axles, of course limitations are involved, and the suspension travel at the upper and lower extremes would be effected.

Fabricating the entire new rear subframe is a pretty big (read that as a major) undertaking though. If a "kit" is available (read all reviews available), I would deeply suggest the "kit" could make life much easier, regardless of who's rear suspension system it uses.