Free standing / unsupported
The alum pipe was unsupported except were it went through the floor and then about another 7" or 8" down to the top of the bilge ballast. Total lenght of the pipe was aprox. 60"
Look for a forum of Pearson 26' commander and ariels and throw out a question to get the exact details from them. I had a 1960's vintage 26' pearson commander (Commander is the big cockpit version) and it has been 5 or 6 years since I thought about it. If I can find a picture I will post it this week.
If you can position the mast step near or on a bulkhead and provide a horizontal frame/strong-back to carry the load to the bulkhead and some vertical frames/stiffeners attached to the bulkhead you will be in good shape.
If you posted some pictures or a more detailed writeup/description of the project you would get MUCH better feedback.