You are most of the way there and I bet a 4bbl and intake will really liven it up. It helps that it isn't a smogger LA - factory says 8.8:1 for a 67 273 2bbl. As mentioned before, some tuning with the distributor advance curve will also help. The AVSII carbs are a good choice and if the 650 works well on my 318, I bet a 500 would be perfect on a basically stock 273.
Please let us know how it turns out.