Rebuild or replace?

Lots of opinions... Here's mine -
It's a convertible. That means it's got structural issues from the ground up. It also means the usage is more about cruising and enjoying than racing around. You want a little more pep - did you ever had a well-built 273 in it? Probably not if it's original and tired. 273s are not cheaper to rebuild properly. Especially the 4bbls. I would spend the money on the engine you have. Do it right, with an eye towards a little more grunt than it had new. Keep everything else original. Maybe change the rear axle ratio to 3.23 if you have a 2 series. While the engine is out have the engine bay refinished in the factory color if it has also been left original. Then enjoy the car.