Will a 1989 dodge ram 1500 3.9L transmission fit in 1996 3.9L?

No, a 1989 Dodge Ram 1500 3.9L transmission will likely not be a direct fit in a 1996 Dodge Ram 1500 3.9L. While both trucks use a 3.9L engine, the transmissions changed significantly between those years. There were differences in bellhousing bolt patterns, output shaft splines, and possibly even the overall case design. You'd run into major compatibility issues trying to install the older transmission.