A watertight GT40.....................the 'holy grail' of GT40 ownership!
One would assume it IS attainable, but 'would likely be very time and $$$ consuming. (The '05/'06 Ford GT's are proof it can be done).
I would imagine the first thing one would have to do is weld shut the 10,000-or-so unnecessary holes in what would be the 'firewall' in a front engine car! That thing is a sieve.
My 40 is also watertight. It takes a long time and many different profiles of rubber seal. I think one of the main problems is where the door tops are essentially flat. The door top slides rather than just meeting edge to edge so if the seal is tight enough to seal there is too much friction so I put a thin wipe of silicone grease on the rubber and on this top part of the door to allow the door to slide shut easily I also have the clips on the inner edges which hold the door down firmly when the door is shut.
My 40 has been on the road since 1996 - never leaked - been in the most torrential downpours on many occasions - I did a host of mods while building, they all worked it would seem! - 17,000 miles of British weather!
I got mine pretty close in the end. Normal rain was fine, but fast at a circuit in a downpour and some would sneak in. Those odd drips add up. But far better than the carpet-is-in-a-puddle where everyone starts out.
But I remember the 'drying out sessions' and wet carpet smell fondly..