Here are some things that WILL make a difference in both torque and HP.
Re-cam
raise compression
polish and port the head (biggest gains here)
go to 2 bbl twin carbs or small 4bbl
HEI
split manifold or Fenton
These are not cheap power gains but unless $ are the big issue you can get substantial power from an I-6. Mine makes 180HP. So your engine already has most of the goodies, just pop the head and start grinding and you'll be surprised. There are cams for grunt and cams for speed so pick what you want just don't expect more than about 5K RPM unless it's professionally balanced, etc.
