Common complaint with an old Rochester QuadraJet carb is the fuel bowls are sealed with two welch plugs (press in freeze plugs) that work loose and leak with time. When you rebuild the Rochester QuadraJet drill and tap the hole for a pipe plug and Loc-Tite it in place.
The 455 Olds carb will be rated at 780 cfm max (not to worry as your 283 will be hard pressed to draw that much). The SBC QuadraJet was rated at 620 cfm and will have smaller primaries to save you on gas, compared to the big block carb. But since you probably have a gas leak now, any change in a carburetor will offer an improvement in mileage.
The beauty of the QuadraJet carburetor is you can not have a too big a carb on your engine no matter how small it is. The QuadraJet only flows what the engine needs by way of it's variable venturi rear barrels, so you can never over carb with a QuadraJet.
The AVS Carter also know as an Edelbrock carb will require a different manifold to bolt to your car as it has a square (equal sized holes) bore. The QuadraJet manifold has two small holes up front and two very large holes on the back. The factory cast iron manifold doesn't flow very well as it was manufactured with cost in mind taking precedent over function (performance). So if you are going to use the Edelbrock instead of the QuadraJet you will need a different manifold. You can buy them used at a greater cost saving as they have no moving parts and do not wear out. (beware of stripped bolt holes, cracks from over tightening and warpage from an over heated engine though). Even an older style Edelbrock or Weinad manifold will be an improvement over the factory manifold.