Bob, I will be interested in how they work out for you. I have considered them in the past but backed off because my bunks are 9' long 2x6, and not supported in the middle. I have been thinking of getting the ultimate boards and bolting them to treated 2x6 for backbone and lowering the brackets two inches. When you get them please let me know if you think this will work. I too have a 1872 SeaArk.
Personally, I wouldn't do it that way. It will cause your boat to sit 2 inches higher on the trailer and you will have to back further into the water to launch and load. You may also have to reposition the winch post on the trailer.
I looked at the ones that have the metal support and decided against them because you would have to cut sections out of it to avoid interference with the bunk supports. I'm going to watch mine and if they sag I'll go to my local metal dealer and get some structural aluminum channel, cut it to fit between the supports and fasten it to the bottom of the boards. I'm just a little over their recommended 48" distance between supports, so I think I'll be OK. Love these little projects.