A 273 block has a stock bore of 3.63", compared to a 318's 3.91" bore. They share the 3.31" stroke, same as the 340. And a forged crank was standard on 273 from '64 thru '69.
I would stick with the smaller 1.78/1.50 valves to reduce shrouding. A pair of 302 swirl port heads and a small port single plane intake like an old Eddy Streetmaster 318 are what I would be looking for. Combine that with a mechancal cam like the purple shaft 276° with .490" lift and it should be a screamer with 3.91 or 4.10 gears.