O2 ?

[daveosx]

Yep on older cars the provision for sensor return from the O2 is not present.
On the ECM circuit board all the grounds are tied together and tied to the case through the bolts.
On the next group of ECMS the grounds were separated to two groups sensor return and chassis.
These circuits are C429 C452 --- C429 is the TPS Coolant sensor Ground and C452 was all other sensors ground.
Starting in 1981 they moved the EST and HEI modules to C486.
In 1991 they started using the O2 sensor return.

On SBC GEN I in all GM vehicles there was one ground point on the rear of the cylinder head.
On the next series GEN 1.5 (one Piece Rear Main) GEN 2 there was the head ground and the Intake manifold ground.
On Later models there was the head, Intake and battery ground.

Internals of the sheet metal GM ECMs all had a common chassis buss and intake buss some had a ground lead for HUD dash.
The cast aluminum ECM cased boards actually had better noise control on the electronics.
Some ECM circuits used a difference amplifier to increase O2 sensor signal reliability it was only then in 1996 that the GM 4 wire sensor could not be substituted with a three wire.
pre 1996 four wire sensors can be substituted with a three wire.

But as far as sensor return lines on pre 1996 GM vehicles the return wires are all tied to ground just some do not share the same physical ground path.
There is an effect called loop induced current that will induce a voltage on a intended shield line or a secondary ground loop.
Electrical engineers at GM had seniority and the electronic engineers worked under them.
The radio engineers worked for AC Delco not GM proper.
So where I or any modern electronics engineer or technician would use shields and ground distribution blocks in a car they did not.

Two areas that will benefit from using shielded cable
The Reference line from a HEI7-8 to the ECM with the distributor side ground only to the shield
Knock sensor with the EST module side of the shield grounded.

pre 1996 The rest of the returns and other grounds will function fine if grounded to the same point.