Extending Frame Depth
I am tinkering with a 62 GT and insulating below the sub-floor and modifying the fresh tank location to below the floor. (was hidden behind gaucho in this model)
My GT came with the belly rotted out, mostly missing, so I didn't have the convenience of a template going back. The best capacity fresh tank I could find is 8" depth. I also custom installed a spare tire carrier into the front below the hitch / tanks. (I didn't like the bumper mounted carrier)
What I'm proposing is to add perforated channel metal (similar to the existing frame in 18ga galvanized with a durable coating) to the existing frame and outriggers. This will extend the "utility space" of the belly and allow me to hang a seamless (read aerodynamic) belly pan to the bottom of the GT. It would need to notch up around the axle, but overall it would run flush to the depth of the axle (no loss to my maximum clearance).
This approach seems better than to add just one bulge to accomodate the new tank. This would not be aerodynamic and would hurt the aesthetic appeal. Also the bulge in the spare tire will be concealed in profile.
After reviewing all the threads on insulating vs not insulating, I have chosen rigid polyisocyansurate (ISO) insulation adhered below the sub-floor and adding louvered vents in the belly pan to provide adequate airflow to dry it out if condensation forms. (don't ask me about dew point i'm not an engineer) Something to do with humidity and divergent temperatures interior / exterior.
My interior flooring will have a 6mil vapor barrier to prevent spilled water (gonna happen) from migrating down to the insulation or sub-floor. Was thinking floating cork which can expand and contract. (per classic rides episode, youtube it)
You can tell, I'm not nostalgic about keeping things original. This is my custom job and I'll do as I please here. I'm looking for flaws in my strategy. Anyone shout them out, I'm listening. Thanks.