new engine

An early 318 (67-69) would be the easiest to just drop in for the 273, as all of the front end parts on the engine will be compatable with your original radiator configuration and they also had the highest stock compression. You could also rebuild it. then it would be a good opportunity to put the larger port heads on it 340/360. If you want to use the larger port heads, then buy some 10.5 pistons, and your final compression with the larger port/combustion chambers will be 9.2 - perfect for a street engine.

Or get a 340 or 360. You can find a 360 much cheaper than a 340, and throw some 2.02" valves and a good cam in it, you will have a good runner (360's can run as good as a 340 if built right). If you go that direction, you can take the current timing chain cover, and water pump off of the 273 and put them on the 360, then use the stock 360 vuibration damper, just remark for TDC as the marks on the 360 damper will be 90° off of the original 273 marks. Then you can use the original radiator in your car.

Or you can run the newer style timing chain, vibration damper, and water pump. Then change to the newer 70 and later style radiator (make sure it is comparable width to fit in the existing bolt pattern).

The best factory exhaust is the 68-70 340 exhaust manifolds. They are around, but a bit expensive, but worth it to save the hassles of headers.