I love those cars, but I agree, not very qwik even with the CJ and 3.91's. A new 4cyl turbo whatever would kick the shit out of that and just about any other "muscle car" except for the biggest dogs of that era.
I don't know how you can say anything from that era with a 428 SCJ wasn't a muscle car. Just like the 454 and 440 powered cars from the other manufacturers were muscle cars.
I'd say its a mid size car. The full size Fords were at least a couple feet longer/wider than that car. Talk about heavy, whats the weight of a 1969 Galaxie?They were faster than the over hyped chevies and were a terror on the race tracks. As for being heavy? Remember that they are a full size car.