That was my first FRP trailer a 1979 SS I loved that thing took it off the frame did a rebuild on it (Not as nice as yours) but I did use the boat liner 3M interior and it turned out great I dragged that thing all over and it never let me down.
Rewired, upgraded interior, put a Bomar boat hatch and an aluminum framed Daylighter RV Skylight in the roof as it was sagging slightly, so the aluminum frame fixed that, plus it looked like a 20' foot trailer inside after as the ceiling and daylight both made it seem so much bigger inside. Put a bow locker on the front "A" frame for battery
and LPG etc.
Great little trailers!