273 warm up!

Problem is a 273 needs to rev to make any real power. And to make use of that power a lot of gears and stall is needed. And with the small bore your stuck with stock valve size.
Heads are the main limit to power potential so porting or notching the the bores for magnum heads would open up the engines capability. And low CR is gonna limit cam choice.

So your choices are just go 4bbl and a mild cam and duels and maybe some gears. And live with a little power boost . Or invest in total redesign of your powertrain at which point your better off going with a different engine.