Fuel gauge stopped working after 04 LS1 swap into 99 truck

We swapped a 2004 gto ls1 into a 99 silverado 5.3 but customer wanted to run his truck alternator (200 amp), all gauges, AC, etc. He wanted it all to work like stock. So I used his truck stock tune and changed all the air, fuel, spark, idle tables to match those of the gto. For wiring, I basically stripped all the gto-related wiring out of the engine harness and repinned his truck harness pigtails (that feed his I/P, MIL, ac request, fuel level, etc) into the gto ls1B connectors. So as far as the stock truck knows it's all the same except engine parameters.

Fuel gauge now reads empty despite having correct resistance (for half tank) between both sensor return and signal wires going into the pcm. Also the ABS light is on now.

Is there anything I need to do to make this all work? Some kind of a relearn with the BCM maybe? VATS is off, btw.

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