273 carb probs!!!

I think 600 cfm is too big unless you plan to spin >5000 rpm. A 390 or 450 cfm would likely be better and should give better mileage and driveability. The 450 cfm I know of is the Holley Economaster (4360, I recall). It is a spread-bore pattern, so the factory manifold would need an adapter. Perhaps another spread-bores like a Thermoquad or Quadrajet would give good low end and all the top end you need. I have one of each to try on my 273.

While at it, the Offy Dual Port manifold is interesting. I bought one to try on my 383.