Dont use 340 heads on a 273!. The chambers are too big, and the block needs to be notched. 273 heads are just fine after a good valve job and a port cleanup. Stay with a fairly mild cam...340 4-speed profile is about max. if you still want some low end.
Theres a '65 273 4 barrel intake on ebay right now.
The dividing point is between '65 and '66. Late '64/'65 intakes are casting #2465726 and '66/'67 is casting #2536563. Go with a 500cfm Carter carb,if you can find one.
Goodluck.
