A note, the fuel injectors ground via the injection computer. They have a constant positive 12 volts when ignition is on and the injection side of the pump relay, energised/closed. They're pulsed via the negative or ground line.

However, you're correct that the manifold does ground the injection computer(possibly the ignition computer too), along with temp sensor, idle valve and maf sensor on the 240. It's vital to ensure these grounds and any grounds between the motor and firewall/chassis/battery are all clean and viable.