[James L Miller]

The 318 BBD 1-1/4" has a larger venturi according to the NHRA specs.
The 318 has 45 more cubic inches, more or less.
The 318 can run the 308 and 576 heads, but may not be an advantage with 1.78/1.50 valves. I'd guess you would end up with the 302 casting heads for either one.
The 273 has slightly more valve lift, about .010" or so.
The 318 pistons are probably easier to come by if you are low-buck like me. Unfortunately, NHRA still lists the flat top piston for the 1968-1971 318s but they came with the four valve notch pistons. I sent in a set of pistons for approval but never heard anything back from Pat Cvengros. Too bad I threw away those 1969 318 pistons 30+ years ago. I looked for one NOS piston but never found one.
The 273 is rated at 162 HP and the 318 is at 182 HP.
It looks like the 318 gets a dual plane intake vs. the flat single plane for the 273.
I'd say the 318 would make about 50 lb-ft more torque, but may not rev as high? Not many guys running the early 318s. There is a guy from NY or someplace up that way running a 1973 Dart with a 318.