99 OBS? you mean its the body style similar to the 88-98 model trucks?

If its what's called NBS which is typically 1999-2007 trucks then the trans will be a direct swap unless you had a V6 before. If it truly is an OBS you'll have to get a special built convertor or a Hughes convertor pilot extension.

Cut the wires out of the harness that weren't needed after you finish the conversion.