The oil sensor for the gauge should be behind the distributor? And the sensor for the ecu/fuel pump should be above my filter?
Maybe not. There is conflicting information. The 1985 - 1991 parts manuals only show two versions of the 350/5.7 engine:
One was the carbureted 350M - RPO code LT9. I think we can forget about that one because in 1987 I don't think available even as an option for a 3/4 ton truck. It had an oil pressure sender (if equipped with gauges) and also a pressure switch that activated the choke heater (and the low oil pressure light if not equipped with gauges). Both were mounted behind the distributor on a common fitting.
The other version was the TBI 5.7 - RPO code LO5. You can verify that this was the original engine (and engine management system) by checking the codes in the glove box.
But even assuming that your engine is throttle body injected, there is still a conflict with what the service manual indicates. Here is the dwg and list taken from the parts manual. Note that it shows a combination sensor (part 10137652) that powers both the fuel pump and the pressure gauge. It also shows the fitting as having just one threaded hole:
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On the other hand, the service manual shows the two sensors in separate locations - and they are reversed from most other applications. Usually, from what I have seen, the sender for the gauge is up top and the switch for the fuel system is down by the oil filter:
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That combination switch is available.
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It requires that 3 leads be connected to it - like this:
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Here is a simplified dwg of the circuit and 3 prong switch:
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So if the fitting that is screwed into your block behind the distributor looks like this:
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but there are only two leads connected to the harness plug, I don't know what to tell you. If you want a functioning oil pressure gauge, it looks like you are going to have to climb under the truck and look around in the area of the oil filter to see if you can find that third wire. It should be tan and - if it is connected to the proper sender - will provide a signal for the dash gauge.
Does the pipe plug above the oil filter look original or like someone recently screwed it in?
If I had to guess, I'd say that originally there were two sensors - not one combined unit. And, they were located as shown in the service manual - with the 2 lead switch up top and the single lead down low.
