I am new to this and I am a little confused. This is what I have going on: I thought it would be fun to build a little hydroplane. So I did. I have a motor for it but I decided I it was not really the look what I wanted. Now I have a 1958 Mark 25 power head and a H tower but I am lacking a lower unit. Can some one please help me figure out what I need. I am unclear of the differences in an "A", "B", "C' and "Q" lower units. I know some had a longer skeg, and different gear ratios. What would be the best choice for me. This boat is for fun only. One more thing, What do you want to avoid when looking for a gear case, welds, shortened skeg, pitting, filler, paint? How do you know what you are getting when you take the chance and buy something you have no way of looking at first. I know these things are very expensive and are very hard to come buy. What is the best way to start, with an empty case and new parts or take your chance and buy one on ebay and tear it down and replace what is needed? Thanks, Ian Kramer