Dartnut said

You hit the nail right on the head here.
When you think of it, the sub frame connectors are thicker than the frame itself. (Or they should be IMHO)
If you submerged a piece of the same material in salt water, it would last for years without eating through with rust.
The modification will out last the car, so why would you worry about it?
As long as it is sealed no water can get in, and even if it did, the only thing that you would maybe have to worry about is a ''sloshing'' sound.
A lot of people over think these things.
Just weld them in, seal them up and paint them, and forget about it!

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