Will a transmission out of 1991 Dodge Dakota 4x4 with 6 cylinder fit in and on dodge 8 cylinder?

No, a transmission from a 1991 Dodge Dakota 4x4 with a 6-cylinder engine will almost certainly not fit in a Dodge with an 8-cylinder engine. While some Dodge models might share some drivetrain components, the bellhousing (the part that connects the transmission to the engine) is different between 6-cylinder and 8-cylinder engines. The 8-cylinder engine's bellhousing will not match the 6-cylinder transmission. Even if you could somehow overcome the bellhousing mismatch, the output shaft and other internal components may be different and incompatible with the 8-cylinder engine's power output and torque characteristics.