There's no such thing as a factory-produced 1989 Chevrolet truck with a 427 cubic inch engine. Chevrolet didn't offer a 427 in their light-duty trucks in that year, nor in many years at all. The largest common engine in those trucks was a 454 (7.4L) big block, and even those were becoming less common by 1989.
To get the firing order, you'd need to specify the exact engine (manufacturer, displacement, and possibly casting numbers). Once you have that information, you can usually find the firing order in a repair manual specific to that engine.
