I don't know squat about EFI, but you are starting with the best Gen I block and heads that are available in my opinion. Long-tube, equal-length headers and a mild cam to match the static compression ratio would be about all I would be able to add. And even at that, I wouldn't know how to tweak the EFI to make the changes work. Use a cam with 0.450" lift or less. More lift than that with stock L31 heads can result in the bottom of the valve spring retainers crashing into the valve guide seals. Of course, there are springs and retainers to cure the problem so you can use more cam lift if you want to spend a little time and money on the heads. You want to know the exact static compression ratio of the motor before you start changing cams.