Need to know what mods you have then look up or get a hold of Walking J he can help you make an informed decision. Or go to his web site and look at the tunes he has available from different vendors. Kojo (member name) has done a few tunes in his day and will make you a custom tune if you so desire.
Look in our vendors section, and you will see HEATH diesel. He does OBD1 tuning for 6.5's as well, and is highly regarded as a great tuner of them. But there are many who do tuning for them, but beware that some of them are pretty dangerous(most of the old chips made by the big name companies should be avoided like HYPERTECH and BD).