Thank you, Phil and Scott D. for taking the time to respond.
I imagine if I purchased a nice laser level I would find many uses for it on the boat and around the house. The price for the one you suggested, Scott, does seem reasonable. I am just not sure this is the best way to use my limited boat budget right now. I spent about 20 minutes rummaging around my garage trying to find a way to rig a plumb bob before I gave up and bought a 'real' ($4.00 Harbor Freight Tools ) one. I am trying to buy the best materials for the boat and rig. I have a hard time doing that and collecting more tools at the same time.
After considering ACX exterior ply I decided to spring for actual AB marine grade plywood for the mast step. One sheet is sitting in my garage waiting to be made into something useful.
I intend to follow the path that Arne made for us regarding mast step construction. I purchased 1/2" plywood. I am planning on 12 layers total to give me a 6" thick mast step. Also following Arne and Phil I plan to make the top 4 layers (2") a separate section for the final alignment.
To Answer your question, Phil, I have no idea if the keel trunk is square to the waterline but it is a more or less straight and flat surface that I can use for alignment.
I am not sure how I would eyeball the waterline on this boat. Should I use bottom of the vinyl boot stripe decal and assume whoever put that on used the true water line? Should I attempt to use the top edge of the bottom paint? It is old and not a straight and continuous line. I thought about using the top edge of the 'scum' (I need to clean the hull) as the waterline. But even using this I would have to assume that the boat was loaded evenly last time she was in a boat slip. What exactly is 'the water line' on this hull?
I watched a video about how to make and use a water level. I will very seriously consider making one. If I can decide on what to call 'the' water line then this would be very helpful.
Scott.