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There are some real experts out there, but not me. Over on Pelican, boyt911sc is one of them, also Timmy2. You'll see them on the thread I linked. boyt911sc may even send you a couple different WURs to try out. He probably knows if the 928 part is good for your application. He can also rebuild your old one.
You mentioned you installed a spare WUR. Each version has its own pressure characteristics, and they are not necessarily interchangeable. Here are the specs and graphs for 1978, 1979, and 1980 SCs. We can get you the correct graph based on the part number stamped on the side. You need the fuel pressure gauges to test. Don't get the Harbor Freight set, they are probably more trouble than they are worth.
The other factor is how the 1980 engine was installed in your 1974. A 1980 has lambda control, so there are electronics connected to the O2 sensor. You will probably have to figure out if this system is working. I think the computer is supposed to be under the drivers seat, but I would start by seeing if you have an O2 sensor and if it is connected.