You may be able to modify the existing tubes to fit your boat. I think the way they make the Monitor is that they start with the unit and four pads that bolt to the transom and then they make tubes to go between the two - adjusting the length as necessary so the unit ends up being vertical. If I remember correctly (the boat is in Grenada and I am not) the tubing is expanded at the aft end to fit over the tubing of the Monitor while the forward end of the tubing just goes into the mounting brackets with a bolt going through to hold it together. I think you would want to keep the expanded part and cut or lengthen the current tubes as needed. I wonder if you could do a mockup with plastic tubing?
I don't think the Monitor is necessarily for a smaller boat than an Aries since the unit is not steering the boat it is steering the wheel. There are some very big boats with Monitors (70'+) it is really a function of how hard the boat is to steer. Our boat is about 40,000 pounds and was no problem for the Monitor (or an Aries I am sure), but was right at the size limit for a Hydrovane which actually must steer the boat. The Hydrovane guy said his unit 'should' steer my boat which is why I went with a Monitor since the Monitor guy said it would steer the boat just fine. He was right.
One suggestion, use very good, low friction blocks to lead the lines to the wheel (tiller?). Don't skimp on these. Also make sure that the leads for these blocks are as perfect as you can make them. I thought my original ones were fine but the chafe of going back and forth thousands of times will show you any flaws. I actually filed away part of the cheek of one block to avoid wear. We find that a set of lines lasts something like 5000 miles. In the instructions they recommend having lines that are several feet longer than you need. As you get wear you can just shift the line back a foot or so and expose a new area to the blocks.
