9V in the harness seems low. The engines utilizing Low Z injectors have a resistor mounted to the firewall to drop the injector voltage but yours should be battery voltage. I would check your harness and battery cables to see if you have voltage drops somewhere, I have found resistance in the engine ground cable down by the AC compressor that made all sorts of issues. I doubt this is your current problem but you should be seeing close to full battery voltage there.

I hope you did not use a Felpro head gasket, I have had a 100% failure rate with them. For these engines I use the later MLS gaskets with good luck.