I build the cab around the floor(using floor as jig) with a few drops of glue on each side to hold it in place. Once then glue on the cab joints dry remove the floor with care. the tape idea sounds good too,,will try that next time.
Good choice to start off with and yes with a few extra parts the snaps end up being nice. As you get more into the glue kits they can be a challenge. Just taken you time and great work.
How I do mine. I'm kinda into the walkins. Say 144" divide by 25 scale is 6". So 6'' into the frame then I add quarter more for clearance of the trailer. I build the frames like he does. Build the frame. Eyeball the rear axial center to the steer on both sides making there even before I make the cut.
If I have a parts box will use frame parts from it.