Will an 1984 2.8l S10 motor and transmission bolt up in a 1991 4.3 blazer?

No, a 1984 2.8L V6 engine and transmission from an S10 will not directly bolt into a 1991 4.3L Blazer. While both are GM vehicles, the engine mounts, transmission bellhousing pattern, and potentially the engine accessories (alternator, power steering pump, etc.) are different and incompatible. You'd need extensive modifications, likely including custom engine mounts, a new transmission crossmember, and potentially driveshaft alterations to make it work. It's a significant undertaking, far beyond a simple swap.