I swapped a 350 TPI into my monte, but with all the wiring harness changes, I don't have a functional SES light so I can pull trouble codes.

Is there any way I can run a wire from the ECM to a bulb or something?

should run a ignition switch keyed (power when key is on) wire to a 12 volt light and then to pin A5 (originally brown/white) of a '165 ECU (MAF computer, not MAP). Any parts store 12v lamp should work. Run a small fuse inline too just for fun (see what amperage the bulb says it uses on the box for the right fuse).

I'm not sure which pin to go to for a '730 (MAP, or speed density) ECU.

So the ECM pin completes the ground to display the light, instead of providing voltage?

I have a 730 ECM but I still have the repinning guides so I can figure out which is the corresponding pin.

yup.. all GM ECMs are that way..