The only reasons Mustang is lighter is because the brakes are smaller, the wheels are smaller, and it has less equipment.
Bring those items up to par with Camaro's equipment and the weight difference would be negligible.
So... on the weight issue...
Take a base V8 Mustang and a base V8 Camaro...
- Remove the Mustang's seats and put in the Camaro's seats - front and rear.
- Take off the Mustang's brakes and put on the Camaro's brakes.
- Take off the Mustang's wheels and tires and put on the Camaro's wheels and tires.
- Remove the Mustang's live axle rear end and replace it with the Camaro's 4.5-link IRS.
- Take out the Camaro's 2 curtain airbags and their associated sensors and electronics, and put them in the Mustang.
Now weigh the Mustang.
What you'll have is a vehicle that is only about 50lbs lighter than the base V8 Camaro, and still has a chassis that's not as rigid.
Add some steel reinforcements (aka more weight) to fix that.
(If I could afford to actually do this, I would!
....anyone got Uncle Donald's phone number?
Why are Camaro's seats heavier? They're larger, thicker, and more comfortable.
Why are Camaro's brakes heavier? They're bigger.
Why are Camaro's wheels and tires heavier? They're larger in diameter and wider.
IRS is known to weigh considerably more than a live axle.
In 2003, the difference in weight between a Mustang's live axle and a Cobra's IRS was 80lbs, and that was a 4-link.
I would bet Camaro's 4.5-link IRS weighs 100lbs more than a Mustang's live axle.
Adding the 2 curtain air bags will of course add their weight.
Obviously none of the Camaro's parts would actually bolt up on the Mustang, but you get the idea.