The schematic for that ecm is very similar to the 1227727, at least for power and ground. Ludis / exatorque site has diagrams. Use Ohm meter to check resistance between C1 and other battery supply points... one of them is connected. Sometimes GM uses extra wire with peak and hold injectors.

Measuring grounds... how? Ground connected on sensor end, ecm connected at other and powered up? Measure from block to wire? For C6 that could mean high resistance in wire between measure point and ground. For C5, problem might be due to C6 not being connected properly. Could also be due to poor connection at ecm. Try pulling each sensor and rechecking voltage?

Wrong way to test is to disconnect C6 and measure to ground with ecm powered up. That would show voltage as ECM is looking for ground connection. Could also throw C5 high if C6 is open. Make sure wire is grounded during testing.

Did you ever get spark to work?