How high area the ceilings? I do 8ft boards on my own, I only struggle at 2.5m+.
Make yourself a T shape prop, about 30mm longer than your ceiling is high, your a piece of heavy string/light rope to the top. Put the prop around a third of the way into the boat gently at first so you can position it, then screw up the other end you are holding. Easier with hop up stands rather than steps. I also use a quick action adjustable prep at the free end and an autofeed screw gun. The props are about 12 quid at screwfix, autofeed gun makes things a lot quicker and easier but I used to do it without.
It WILL take a fair bit of practice to get the knack. If you can’t mange, just half them.