My mast support can be altered (I added an extension myself to the mount where the winch is for a regular boat). Anyway, it used to clear the box on my half ton, but now that I have an Xterra, my mast does not clear the roof.
I was too much in a rush when I noticed, so I let the mast stick out further behind the boat so that it would clear the rear window and put a red flag at the end sticking out to be legal. But I only tow my boat twice a year, in the spring to bring it to the lake and back in the fall to bring it home. I'll fix it before bringing it home.
One way to extend it is to find tubing that would fit over the existing tubing. Cut the post, slide the extension and bolt it in.
1978 Hobie 16 Keoke, sail# 36 84