I predict worse mileage with the 273. A small motor trying to push a big lump equals poor mileage.
You'd be better off buying the LM1 someone mentioned and tuning your current 360. 360's can get good mileage, but not with an off the shelf carb, especially a Holley generic type carb.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
