The bulkhead connectors changed in 1989 BUT the 1987 5.7L TBI setup can be used to power the 454, you just need to change the injectors and the Chip in the computer. (unless you are using the 4L80E.
With that said, you can unpin the bulkhead connector and pin in the 1990 wires into it if you need more length. Is there a reason why you didn't use the 5.7 Harness?