I'm in the process of restoring a 74 Tsunami,and just removed the vent boxes you asked about.
They are plastic,and are bedded directly to the hull,not the liner.Mine were attached to the hull with big sloppy globs of what looked like some putty made with some thickend resin.
They didn't come off easily,i had to use the Roto-zip,and a chisel to get the globs of gook off,so i could remove the boxes.Also keep in mind,that i had the rear cap,and inner stern liner removed.I don't know how you'd be able to easily remove them without doing this.
Maybe the following photos will be of some help to you.
ready to be removed,you can see where i had to crack the thickened resin.
starting to remove the box
box now removed from hull
Showing liner cut and starboard side.
Good luck with your project.