Its been said many times before. But if you are reusing the 273's pistons, wrist pins and rods then any Poly 318, or forged 318/340 crank will fit but still should be balanced.
It is true that in 1962-63 when they were doing the engineering of the 273 they used the Poly 318 crankshaft and connecting rods as they had them all tooled up and on the shelf.
But the mass of a Poly 318 piston bob weight is a lot more that the smaller bore 273 so they tooled up some very thick walled wrist pins to match the Poly's bob weight
But if you want the best, replace the 273's HEAVY wrist pins with some light full floating pins, do the David Vizard's crank weight reduction trick and rebalance it. They will be able to remove over 6 pounds of rotating mass