With a big block that old and towing 15K pounds, think in terms of 5-6 MPG. If your going that old like I said your only best choice is a 5.9L Cummins. Ford 7.3s were pretty good, but still not as good as the Cummins. GM never had a reliable diesel motor until they partenered with Isuzu to market the Duramax. Gas motors can be made to work, but going 5 years newer and getting a diesel would be far more satisfying.
If I remember right the 2000 GM duallies only had a 10,000 GVWR, maybe even lower. Leaving you exactly zero room for towing.