This makes much more sense than the idea that the 71-73's are 150-200 lbs heavier than the 69-70's. The story is that the 429/460 big blocks weigh about 140-170 lbs more than the 351 C. So, if you car had the 429/460 it would have weighed probably 35-65 pounds more than your friends 69-70, which is not a big deal, and the 429/460 engines are heavier than the FE. I would be willing to bet that that if you took 2 empty shells, one of a 71 and one of a 70, the weight difference would be less than 100 pounds, would not be surprised if it ended up less than 50 pounds.