Start here:
SPF GT- Water/air sealing, the "DIY" thread - GT40s.com
The GT40 has a lot of potential water leakage areas that will need to be addressed to resolve this issue. It is not because the chassis is poorly built, it is in the nature of the design.....these were race cars and back then, drivers got wet. You can eliminate most of the water intrusion but as I told 1795, they are "water resistant," not "waterproof"!