273ci--a keeper in my 64 Dart GT 4-speed?

I'm learning from experience that rebuilding a 273 can get expensive and there are far fewer options available in terms of pistons, head gaskets, and cams. Not that you can't get everything you need, but there are fewer choices and things tend to cost more. That said, I really like original cars so I would do the same thing again.

I recommend the Rebuilder's Choice domed pistons from Kanter Automotive. If you run the numbers you'll find they don't make nearly the advertised compression which is a good thing if you plan on running pump gas.

My tired 273 automatic with a 4 barrel would only get like 15 mpg on the highway. I'm hoping that will improve with a fresh engine, higher compression, and possibly some attention to carb jetting.

If you do swap to a 318/340/360, hold onto your 273 as it will increase the value of your car. You'll probably also want to replace the stock 7 1/4 rear end for anything beyond a stock 318.