1991 f-159 4.9l cranks but won't start fuel pumps in both tanks loud

You may be right on your hunch.
The fuel pump gets power from the pump relay for 3 seconds when you turn the key on, then no power until the engine starts. If the fuel pressure is low it could take several trys to get the pressure up to normal and the engine start.
A mechanic would check the pump relay and the fuel pressure with a gauge before and after cranking the engine.

Relay, on top of fuel tank under parcel shelf, check its getting power, if not its fuse, if it is, bin it get a new one.

Yes, it should. If it is conventional (like other GM cars), the pump will turn on for a couple of seconds when key is turned to on. This will prime the system for starting. When key is turned to start (cranking), the pump is turned on again at the relay, and pump stays on as long as cranking the engine happens. Once engine is started, the oil pressure switch keeps the pump running.
The best way to check if pump is good is to use a fuel pressure gauge. If it does not have a test port called a Schrader valve on the fuel rail, the pressure gauge must be teed in somewhere convenient. But if you take off the line at the filter, some gas should come out with key on, and when cranking the engine gas should come out in a steady stream.

If you haven't replaced the fuel filter in the last year or so, do it now-basic maintenance.
Check if the pump is working by turning key to ON (not crank), and listen for the pump in the tank to come on for a couple of seconds. Should hear a short hum or buzz verifying tank is working. If you don't hear it come on, either the pump is bad, or power is not reaching it. May have to pull the pump connector off at tank, and check for 12 volts on pump wire when someone cranks the car. If power there, the pump must be bad. If no power there at pump, check fuses for pump and fuel pump relay.

It sure could be. But, first...by "won't turn over" you do mean it cranks at normal speed but won't start?
A simple test of the fuel pump is to stand by tank and listen as someone turns key to on. When first turned to on, your pump will activate for 2 or 3 seconds, pressurizing the fuel system, and then shut off if the engine computer doesn't get a cranking signal. Do you hear it come on? If not, then something's not turning your pump on. If you can hear it buzz on briefly, then maybe it's your filter. Another test: Spray a little starting fluid into your intake manifold (remove the big hose going into intake manifold) while someone tries starting it.If it doesn't fire and try to stay running, then it's probably an ignition problem. But if it does fire off, then you know it's a fuel problem-likely your pump or filter. good luck.

Had friend put OBD diagnostics to it and error code indicated fuel pump ????? No code indicates replacing any part . Indicates testing ! pressure and flow testing , voltage drop testing the electrical circuit etc... There is a check valve in the fuel pump assembly that is suppose to keep pressure in the fuel lines an fuel rail . When the check valve goes bad allows fuel to drain back into the tank. Try turn the key on an off a few times to build up pressure . The PCM energizes the fuel pump relay for two seconds at key on .

If the car will not crank then you have a battery, cable or starter issue. If the car will crank but not start then the problem may be the fuel pump relay. There should be an electrical diagram on the side of the relay. Apply 12v to either side of the coil and you should here a click. Then check for continuity between the other two terminals with voltage applied. If you have no continuity then the relay is bad.

Check for fuel and spark. Can you hear the fuel pump buzz for about 2 seconds when you first turn the key on? Might have a person take the filler cap off and listen.
If not then you should check the fuses and the relay under the hood in the fuse and relay panel. Turn the key and have someone listen or feel the relays for a click. You should also be able to see what relay is for the fuel and swap it with another one that is in there.

I always tend to lean towards fuel because the pumps will go out, many times due to people running the tank low on fuel.

Is it getting fuel? Have you tested the fuel pressure? You may have a faulty fuel pump or fuel pump relay. Can you hear your fuel pump buzzing. Remove your gas cap and have someone turn the key to the run position while you have your ear near the open gas tank. You should hear the pump making a buzz. If you don't hear anything you have a bad fuel pump or problem in the electrical circuit to the pump. If you do hear it running you still need to check the pressure. If the pump is buzzing it still may not be working correctly. You could try giving it a shot of starting fluid and if it runs on the starting fluid you know you have a fuel related problem. Be careful with the starting fluid because it is very flamable.

Fuses mean short somewhere.Fuse is always live it feeds relay but in tank pump [ fuse#4or#6 i forget anyway visa versa ] is not live until this.Relay clicks once when key tuned to on and once more when key turned to crank engine it is under dash by glove box and loud enough for someone with ear at this spot to hear.In tank pump should not be live until engine starts to crank.