For the record I have had a few faulty rev switches in my day.

If you will PM me I will send you a line drawing of the circuits for a 7F, non TWC truck, and a couple photos I took of a 7F truck in our shop that happened to capture the rev switch in view.
The rev switch is an orange/brown colored little plastic box found only on non TWC 7F models. The box is situated behind the left step insert panel. It may be the same or similar set up on 6F trucks.
The later 7F trucks with TWC and ECM do not have a rev switch, the ECM gets the tach signal on those.

But you need to make sure the tach signal wire is intact and sound before jumping to replace a rev switch. To my knowledge, we have never encountered a failed rev switch. (But I bet we will now!)

This tach signal goes to a relay? Just wondering where this might be. Sounds exactly like old Volkswagons....they has a special relay that powered the fuel pump as long as it had a tach signal.
I'm just scratching my head at Toyota for making it so hard to find manuals. Even the parts manual you buy from them has only "reference' part numbers that are useless to you. Toyota is big, but not user friendly!

I do not have a wiring diagram for a 6F with 4Y.
But I do have a diagram for an early 7F (pre-TWC).
The fuel cut relay gets its ground from what Toyota calls a "rev switch". My interpretation of rev switch is that it is a switch that closes when it senses engine rotation by a signal generated in the igniter unit inside the distributor. Essentially, a tach signal circuit that tells the rev switch that the engine is running (or at least cranking) and thus it is OK to close the fuel relay and operate the fuel solenoid. This way, if the engine quits running for any reason, the fuel solenoid is not able to let fuel escape that could pass through the dead engine and, well you know why that is bad.
Check the tach signal wire in the small, two wire shielded coaxial cable that serves the distributor. If the tach signal wire is broken, you can have stalling or no start.
If you find a broken tach wire inside the coax cable, and you decide to splice repair it, make sure you DO NOT get any strands of the bare shielding wires included in your splice.
If the tach wire has continuity to the grounded shielding wire, you will not have spark or fuel.

It is an older model, had a quick look at it. It's not dual fuel. If there is no ECM, what gives the negative to the fuel cut off? Is it the oil pressure sensor?

"ECM after a tach signal from coil?"
Does this 6F truck have an ECM? If so, that would be news to me.
I have been under the impression that TWC and an ECM were introduced with the 7F models, and I may be wrong.
Was this truck built as a dual fuel truck?
We used to have some 6FGCU25 trucks in our fleet and none had TWC and an ECM, They all had the simple Aisan vacuum fuel lock regulator which had only 1 main fuel solenoid valve on it.