I wouldn't count out the brakes and such. Redfish makes some good points, as do many other folks on here. And to be perfectly honest, headers aren't a requirement. If you just want to continue to enjoy the car and aren't really looking to set the street on fire with it, even mild cams still breathe just fine through stock manifolds...I just like many others, dig the look and sound of headers.
Exhaust doesn't need to be overkill...dual 2 1/4" is more than enough even for a modified 273 breathing through stock induction.
Moving up to a 4bbl will liven things up considerably over the little Carter BBD though. I can't say the later model iron intakes would be a bad idea, just gotta make sure of the fitment and port size...I'm probably wrong here, but the 360 4bbl intake ports may actually be bigger than the stock 273/318 ports. I'm pretty sure that engine should have 675 casting heads...also shared by 318s of the same vintage up to 74...those are open chamber heads.
The rear end/gear swap will really only be needed if you beat on it with modifications. Stock 2bbl 273 won't overpower that rear unless you're just downright abusive. My stock 40+ year old 7 1/4 sits behind a 4bbl'd 5.2 Magnum...I just know not to drive it like a jackass. That being said, if you want to up the ante for power, an 8.25" out of a 73-up A body is a bolt in affair, but you'll have to shorten the driveshaft about 1.5" if memory serves and anything stock will come with 2.45 highway gears, but the pieces are readily available to make it into a SG with pretty much any performance ratio. Otherwise, an 8 3/4" or even a Ford 8.8 can be made to fit (and that swap, if sourced from an explorer, will come with free disc brakes, trac loc, big bolt pattern and 31 spline axles)...there's even a sticky for that, and they often cost less to purchase, set up, and install than an 8 3/4.
SO many possibilities...as mentioned at least a few times in the past few pages--nail down what you want out of the car first...then work toward that goal smartly. A lasting home is built on a good foundation, your car should receive the same gratitude.
