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You said you had pulled the line from the pump & it was pumping, so was it an electric pump mounted on the frame crossmember just about under the driver's seat? If so then that is a stock setup. The question now is how did you power the pump to test it, since the ignition must be in start mode or the engine running before the fuel pump relay will supply power to the pump?
The truck runs when fuel is poured through the carb, so you need to start at one end of the fuel supply system and find where the flow stops.
Get a long enough piece of insulated wire to reach from the battery to the fuel pump plus another piece at least three feet long & four small alligator clips to make a long jumper wire & a short jumper wire, each with a clip on each end. First change the fuel filter, they're cheap and that will eliminate a dirty filter. While the filter is off blow through the fuel line towards the tank to make sure it's clear. You should also be able to hear the fuel bubble in the tank. Next clip one end of the long jumper to the electrical connection on the fuel pump, then remove the rubber fuel supply hose from the carb and stick it into a container to catch any fuel. Now attach one end of the short jumper to the positive battery post- don't allow the other end to contact anything- then away from the battery connect the two loose jumper ends together. Disconnect the jumper ends away from the battery after a bit. If you get a strong steady fuel steam into the container the problem is either within the carb or is electrical. If you get a weak or no fuel stream then it's probably the pump. Leave the fuel line disconnected from the carb, remove the jumper wires & test the pump.
To test the pump-
Hook one end of the long jumper to the positive battery post- don't accidentally touch the other end to anything. Hook one end of the short jumper to the electrical connection on the fuel pump, extend the other end away from the pump. Pull the rubber hose from the hard fuel line on the carb side of the pump and stick the end into a container to catch any fuel. Now well away from the pump and container connect the two loose ends of the long & short jumper wires. The fuel pump should immediately start to run. There should be a steady stream of fuel the size of the line with a bit of pressure into the container. After running the pump run for a bit break the connection away from the pump & remove the jumpers. The pump should be very quiet with a very slight vibration. If the fuel flow seems weak or the pump is very noisy or you can feel it thumping the pump is either full of dirt or failing or both. You maybe can clean the internal pump filter, if that doesn't work replace the pump. An aftermarket 4-7psi pump is fine, they are $50-$60.
While the supply line is loose at the carb & pump ends blow through it to make sure there is no blockage. Re-connect everything, you should now have fuel to the carb, you can test as before if needed.
If you have good fuel to the carb but it still won't run you can bypass the pump relay by jumpering the pump as before while trying to start the truck. If it starts & runs only with the pump relay bypassed replace the relay.
If you have eliminated e carb report back here, there are still some things to try.
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'78 Chevy LUV,1.9,4sp,headers,31's,SAS Dana 30,Dana 20,SOA rear w/Aussie Locker, rough body, bought new 12/4/78.
'87 Trooper,2.3,5sp,headers,31's,ball joint flip & spacer,Aussie Locker rear,Superwinch hubs,brush guard w/5.5K winch,more to come.
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