This is my mancave closest and projects in a various stages of started. Some projects at 90%, 50%, or maybe all the halves glued together the point is that I have condense my area substantially... The garage storage is basically the same may be 75% cut down but saves a whole bunch of room.
Now unless you have the time you would never be able to cut all your boxes down I usually do them a couple at a time, or when you first start a kit. I have to say though most kits will fit in a cut down box and some you may have to cut the parts trees down as well in order to get them to stack within the box with the body and the body will have the interior and chassis in place.
This saves so much room and can be on a rotating bases when you finish up builds.

To most this seems overwhelming and a lot of work but if you do a little at a time it will go fast. Hope this helps