Will a Chevrolet 350 engine fit 1989 4.3 liter truck?

No, a Chevrolet 350 (5.7L) engine will not be a direct bolt-in replacement for a 1989 4.3L (262 cubic inch) V6 in a truck. While both are small-block Chevy engines, there are significant differences in the engine mounts, bellhousing bolt pattern, and potentially the oil pan. You'd need significant modifications to make it fit, including custom motor mounts, potentially a new transmission, driveshaft, and possibly exhaust system. It's a major undertaking, not a simple swap.