I wouldn't worry about pulling to 5 g's. 273's are a tough call because they have their limits. You are pretty much there. You can put a cam that works to 5000 and springs to match but your heads can't handle a ton of rpm. Too big of a cam turns it into a dog at lower rpm where you need it the most. There's some good running 273 cars out there but probably not much more than yours. The very few A body race cars running 273's are pretty serious pieces. They probably run 13-1 compression or more with .650 lift cams and big heads with a lot of machining to go along with. Like I mentioned, contact a few cam companies and see what they recommend.
