Go ahead and wire brush those studs and nuts, then douse them in penetrating oil before attempting to break them. If you've got a torch, heat cycle them a few times too. Should improve their chances for survival.
Pull the cluster, disassemble it as per the Pelican tech article, cleaning rust as you go. Sandblasting is the quickest/easiest. Repaint all parts in rustproofing paint- I use POR-15. Be sure to rebuild with the bronze bushing kit- 10 to 15 bucks is worth it.
Your brother's right- some drips find their way here- but the more common reason is that a leaky master cylinder starts the rust process.
There are ABS plastic floorboards available, or you can fabricate a spiffy drilled aluminum one...
Let me know if any of your pedal cluster parts don't survive; I've got assorted spare parts from an assembly I picked up. Be happy to send whatever you need.
-d
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Dave
'72 911T to '73 RSR Replica Project