witch are the best flowing 273 heads#

All LA heads will interchange*. But there are 2 different port and valve sizes:
273/318s have small ports with the smaller 1.78/1.50 valves, as well as smaller combustion chambers.
340/360s have the big port heads with 1.88/1.60 valves** as well as larger combustion chambers.

* Big port heads will bolt up to the 273, but the 273 block will require notches for valve clearance.
Which leads to shrouding of the valves.

Hope this helps:)