@Richard Sweet, the 675's are open chambered 318 heads. Same valve size as the 273 heads but the combustion chamber is as large as the cylinder. A loss of compression and power. You didn't mention whether your 273 was a 2 barrel or not or whether you will need to bore it and replace the pistons. Here's a list of closed chamber heads that will work well on your 273. The #302 heads are from the Lean burn 318's in the late 80's. They have a closed "swirl port" combustion chamber (Very similar to all the 273 heads) but they have hardened valve seats which are good with non leaded fuel.
64-65 #315 closed chamber 57-64.5cc
64-65 #080 closed chamber 57-64.5cc
66 early #178 closed chamber 57-65.5cc
66 late #234 closed chamber 57-64.5cc
66-67 #920 closed chamber 57-64.5cc
68-69 #675 open chamber 60-67.5cc
68-71 #675 open chamber 60-67.5cc 318 LA
87- 90's #302 closed chamber 56-65 cc 318 LA lean burn
If you are going to bore it I would suggest Egge HP pistons or the domed pistons from Kanter. There may be another place or two to get them. I'm afraid if you used a low compression flat top and open chambered heads you will be disappointed in the combination. Here's a photo of the difference between the two. I wouldn't be concerned about the carb. 500 cfm is fine and 600 works without overcarbing. I get 18 mpg/hwy with my 66 with a 3.23 gear and a fairly tall tire.
Here's open chambered v/s closed chambered:
