Newbie here starting my first A body

Usually you don't need to replace the whole rear quarters due to the rust, just around the wheel arches. Unusual that the whole quarter would be rusted and not the bottoms of the doors and lower front fenders also, plus the trunk wheel well. Where the roof meets the quarter panel is another area, often with total rust thru, especially under a vinyl roof. There is a brace there that holds water. The cowl bottoms can be totally rusted if someone let leaves build up there.

I thought that any 318 intake manifold will fit a 1966+ 273 car. At least the bolt holes are the same angle. I recall the ports are the same width too. Since the engine runs fine, I wouldn't put my first effort there. You might get used to the high revving engine, and it does give better mileage. 4.5L is hardly small today. Many Euro cars have much smaller V-8's and cost >$50K.