273 from a 1964 Dodge Dart GT

Keep the steel crank.
You could use it for the super charged app.
318 were cast, strong, but not as strong as the steel version.

plus...your torque convertor is the small snout, early style and unless you have a new one made using a later case... there will be a mismatch of small snout / big crank register... may cause issues like vibration.
1968 was the change over, everything before will be small snout/19 spline as opposed to large /27 spline iirr.
Using the 64 trans right?
Keep the crank or buy a late case/early inerd convertor. Machinist should have known this..