Will a GM 1986 4.3l motor bolt up to 1984 s15 high sierra?

No, a 1986 GM 4.3L (likely a small-block V6) will not directly bolt up to a 1984 S15 High Sierra. While both are GM vehicles, the engine mounts, bellhousing bolt pattern, and potentially the transmission compatibility will be different. You would need significant modifications, likely including custom motor mounts, a new transmission (or an adapter), and possibly other modifications to the engine compartment and drivetrain. It's a major undertaking, far beyond a simple bolt-on swap.