1975 Ford F150 Wiring Diagram Circuit Number Chart

I hear you about parts guys not knowing much about the older vehicles. They just don't seem to keep that information in the computers. That new relay certainly looks correct & the parts number matches so it should work fine. At least now we know we have good contacts in it so one less thing to worry about.

Now I have to get even more specific. You wrote:
"Cleaned up a small spot on the frame that the tank actually attaches to, used an alligator clip as a ground and tested the leads on the wire harness at the tank side."

The "wire harness" part of that is a little confusing. Do you mean the connector at the tank that has two wires? It looks like it from the picture.
IF the wire you are checking is the one going to the Gas Gauge (Yellow with Red Stripe) and you are getting a reading of 1.27 volts pulsing, that is not unusual and does not indicate a problem with that circuit, Frankly you need to ignore that wire in trying to track down the fuel pump issue.
The fuel sender/gas gauge circuit is completely separate for the fuel pump circuit. it has nothing to do with how the fuel pump operates.

The wire you need to be concerned with that feeds power to the fuel pump is the Black with Red Stripe wire. If that one is what is reading 5.94 volts*, that's a little low for pump voltage, but the pump should still work. Fuel volume & pressure should be good enough to run the engine.
*See Random Thought about Battery Voltage below.

Did you test voltage in this wire with the oil sender disconnected or connected? Did you have the key on "RUN"? Please test it with the oil sender disconnected & the key turned to "RUN" and see what voltage you get.
If it's still around the previous 5.94 volts, I'd suspect the fuel pump resistor is out of spec, & deserves a retest.

Again, don't be worried about the fuel sender side of this, it has nothing to do with the fuel pump circuit. The voltage going down from the gas gauge power feed to the fuel sender ground is normal.
The fuel sender is basically a variable resistor, so it is going to act like a voltage drop device, which is exactly what it is designed to do. We can get into the fuel sender later if you want, but we need to make sure your fuel pump circuit is right first.

Picture 5 - Fuel Pump Power Feed Terminal under Flange. If the tank connector is in it's place on top of the Flange, to check for voltage at this point you need to have the oil sender disconnected and the key on "RUN". Otherwise you will get a 0 (zero) reading. More on this next....

If you checked resistance of the oil sender with it's wire disconnected, and it still read infinite resistance (screen going blank), that means the sender is in an "open" condition (no voltage can flow. That does mean the sender is bad.
The oil sender is supposed to be in a normally closed condition (voltage can flow) when the engine is not running (no oil pressure).
Again just to be clear on testing this item - Meter set to 200 Ohms Range, Black Probe to body of sender, Red Probe to Terminal of sender. Here's a pic of one for reference:

20R Oil Sender

It would more than half-way suck if all along the oil sender has been the source of the fuel pump problem. But it's bound to be original (these things almost never break) so replacing it with a new one is still a good idea. They do not cost very much at all.
Most parts stores will not have them in stock due to the age, but some will have them listed in their databases and can special order them (usually charge you excessive shipping too!). If it were me I'd just order one up from Rock Auto:
https://www.rockauto.com/en/catalog/...+/+switch,4588

Either of the 1st two shown are good units (WVE - Wells Vehicle Electrics & Standard Motor Parts), I prefer STANDARD brand parts because they have never let me down.
IMPORTANT NOTE - Only order/buy the oil sender with the SPADE TERMINAL. The "round type" of sender shown with the "button type" terminal will NOT work.
Also, these senders usually come with thread sealer already on them. Do NOT add any extra thread sealant when installing.
I do not have an install torque spec for this part. What I do is thread it in by hand using my finger until it stops, then give it 1/4 to 1/2 turn with a wrench, just like an oil filter.

* Random thought - Have you checked your battery's voltage? Your comment about the 11.4 volts on the Yellow with Black wire makes me wonder a bit. Check the battery with your Volt Meter. If the battery reads anything less than 12.5 volts, it's either in need of a charge or the battery is bad.
A battery with lower than normal voltage is going to "skew" all your meter readings down the line.

Let's not get too far ahead of ourselves on external fuel pumps. Get that new oil sender in & retest first. As long as the wires in your harness are intact it should all work as designed.

I do appreciate that you are thinking ahead on alternatives though. I have seen some people with this model truck resort to the external pump, some of them doing it "wrong" though. Just basic info on it. There is no need or reason to mount an electric fuel pump in the engine bay, and you are right, it's a safety & fire hazard to do so. Besides, electric fuel pumps work best when mounted/placed as near to the tank as possible. That way they only have to "draw" gas through a very short length of fuel line, and as long as the pump is decent in quality, there should be no loss of fuel pressure at the engine side. The hardest amount of work the pump has to do is the "drawing" or negative pressure side of it. That's one of the reasons why Toyota (and a lot of other vehicle makers) placed the pump in the tank. The other reason is that the gas actually acts like coolant and lubricant for the pump.

Keep at it, you're going to get this to work one way or the other. I'll still be here. PS Your inclusion of the pics is very helpful, thank you for making the effort!

Source: https://www.yotatech.com/forums/f114/1975-1978-pickup-electric-fuel-pump-wiring-diagram-info-309287/

Posted by: lesterteneryer.blogspot.com