Okay, since we ARE a bit "Off Topic", check this link out. it says that the road runner weighed "3436lbs" . I didn't remember how much myself, but I sure couldn't see how it could weigh as much as a "Crown Vic"! 
If you have any knowledge about the concept behind the road runner, you would know that the cars originally had nothing that wasn't needed and was built to go against the GTOs and 442s (and with thoise cars, you had to pay extra to get more performance, while, with the road runners (and Super Bees) you paid extra for the "convenience" stuff. Sure, if you wanted the 440 or Hemi ypou had to pay extra, but the 383 was a pretty good runner as it was. The road runner was about the same size of a car as the Crown Vic and the the Crown Vic is a 4door car, which adds weight as well. I don't know where you got your figure but it is just a bit off.
Not trying to get into it, just trying to enlighten. 
http://www.conceptcarz.com/vehicle/default.aspx?carID=1623&i=2#menu
http://musclecars.howstuffworks.com/classic-muscle-cars/1969-plymouth-road-runner-hemi.htm
that's where i got the curb weight of 3940. it does say its the hemi tho. i'm sure thats good for another half a ton or so lol (j/k, huge motor with huuggge heads). back then cars where just made out of thicker metal, every exterior part was made out of metal (that's how they got away with more or less flat body panels, and now all cars have body lines convex or concave shape to them), and to top it all off an iron big block up front.
i'm just used to typing crown vic because i'm on a crown victoria forum too (i own one). i'm the complete opposite of you on this i don't see how my car can weigh as much as a '69 road runner. its a big car but its got an aluminum engine, aluminum drive shaft, 4 wheel disk brakes, thinner body panels etc. i'm probably wrong tho, i'm a rat rod kinda guy, not a muscle car guy.
Yeah, I hear where you are coming from. Something that needs to be remembered also is the fact that the cars nowdays use a lot of plastic and thinner metals and such, but also they are designed for areas like the front end, to collapse to keep the damage to the passenger area to a minimum and that is gonna require some heavy stuff as well. Cars back then weren't made that way. Now, if you want to see something that was
heavy, I had a 74' Dodge Royal Monaco (ex State Police car) that weighed in at nearly 5,000lbs! That car was alot bigger than the road runner! It was PUSHED by a 440 Interceptor and would run pretty good, considering the weight of the car. I mean, when you pushed the gas pedal down, that car left, and folks usually paid attention because it was still blue and white, with the spotlight and the small hubcaps with the holes in them. Got alot of reaction when I'd top a hill AND THEN, they'd realize that it wasn't in service anymore as a cop car!
I had wanted to pull the engine out of THAT one and put it in a road runner! Guy that bought it off me, was gonna put that powertrain in a 56' Dodge stepside truck and show it to me when he was finished. Never saw him again, so don't know what happened? Also, about the road runners and Super Bees, initially, the cars were bare bone with nothing in or on the car, that didn't have to be there (except maybe the heater and a radio). the base runner even had "taxi cab" type interior. No Frills!
