You'll need to find the specific engine code (usually a letter/number combination on the engine itself) to get a more accurate horsepower figure. Even then, the exact HP could vary slightly due to manufacturing tolerances.
However, a *general range* for a 1990 Chevy 350 in a truck would be between 170 and 230 horsepower. The lower end would be more likely for carbureted versions, while the higher end might be seen in fuel-injected versions.