Well, the sky is the limit... That is, 350 Chevy responds to modifications very well. Tons of goodies out there for it, as well.
Start by defining as clearly as you can, what your performance goals are. Next, establish a budget. The budget must be realistic, both from an ability to maintain it and the goals. You can't get 500 horsepower for $400...
Decide whether or not to build the engine currently in the car, or pick up another "core" and build that. With some minor variances, a 350 Chevy is a 350 Chevy. Your '89 uses the "late model" 1-piece rear main seal. It's easier to use that configuration than change everything over to the earlier versions.
A 4-bolt main block is desired for performance. You should consider a "stroker" (383) to achieve the most "bang for the buck". That may require upgrades to the injection system to feed the larger engine. The computer will also need upgrading, HyperTech and others make the "chip" for the changes.
The drivetrain will need to be upgraded to take the increase in power. The transmsiion should have a performance rebuild including a "stage 2" shift kit (if automatic). The rear axle may become the weak link in the chain.
Define your goals and approximate budget and we can get specific!
Jim