No I don't use priority in my lists at all in terms of having them ordered in some priority. I use only when deciding on the thing I will work on next as I look at my lists of next actions.
For me for that project it would be in someday/maybe because I know that we are heading into the last of the summer quarter and I don't have time to do any building projects like that. I might add to it that I need to clear the space and measure as support materials. The way I handle my Someday/Maybe is that I have a not in DEVONThink "Main House Projects to Do" and in that would be the line "Hanging Storage in Garage" and maybe under that a link to a few places I've seen or a reference to a picture of what I want etc.
I swap out projects quarterly on the solstices and eqinoxes. So if I can't get any action done on that project in the current quarter it would be parked in someday/maybe. A lot of my projects have individual action items that can only be done in a specific season and in some cases there are actions that need to be done in different seasons or so man that the project itself carries over forseveral years. That is unusual for GTD. What I do now to keep the current projects to a reasonable number is to put the on-hold projects backinto my someday/maybe system until my next big quarterly review.