Looking to replace my 273 with a 318, which one?

You can use your present 904 and converter. You need a bushing ring to take up the space between the smaller / older converter snout and the newer / larger crank recess in ANY of the later engines, whethe 318/ 340/ 360

So far as balance, you can get a B&M flexplate which is balanced for use with the 360 using your old converter.

727 is different overall length from the 904, and has a different output spline, will need a new driveshaft.

So far as buying crate engines, I hope you are aware of the differences between LA and magnum engines. Both came in both 318/ 5.2 and 360/ 5.9