Several years ago, with the engine out I did several things to reduce the heat impact on the passenger area.
- Power coating of the header
- Wrapping the parts of the header inside the engine bay
- Stick-on heat insulation to the bulkhead and down the trans tunnel
Since it was all done at the same time, I couldn't say which had a greater impact, but it was a big improvement.
As to moisture, the car stays inside since I don't put the hood on I avoid the wet.