You'll need to measure the bolt configuration (2 holes vs 4 holes, front to back vs top to bottom, etc.) on your boat now and then do the same for the towers your interested in.
There is no standard in the tower market and each company does their own designs so it will be some good luck of you get an exact matach.
If the new base is larger than the old one then it would cover the old holes. In this case you would want to fill the old holes then I would recommend fiberglassing underneath to get the strenght up in this area.
(Message edited by monstertower on May 28, 2003)