crosswind air gap or stock 4bbl manifold

If you're not going to stick with the stock 340 spread bore intake, then I would say the best one for a 273 is the old Edelbrock D4B for the 64-65 273, or the Edelbrock LD4B for the 66-69 ones.

They pull great off the line and will give it some top end also... And will match the stock ports better if you didn't already gasket match them to the 340 intake...