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

Same question...why change?? A 273 and a 318 are the same motor...well almost. The 273 will have a mechanical cam and lifters with adjustable rockers. Other than that, they are almost the same, and you can probably build your existing motor for less than the cost of a crate motor!!

I've seen 273's run 10 sec qtrs, so I know they have the power capabilities!!

But, to answer your question, any LA 318 will bolt up, although you will need to change your converter as the 273 converter has a smaller snout and will wobble in the larger crank hole of the 318. The exhaust will bolt up, as well as everything else like pulleys, brackets and such. Now, a later Magnum style...I'm out on that one, maybe a member with more knowledge of the Magnums will chime in!! Geof