Just spray oil in the inside of the frame rails. The welds will hold up for years to come... it’ll be fine.
No need to scare the guy over nothing. It’s an old truck that is being kept on the road. It’s not going to break all of a sudden

Looks better than what you had before. As I said in another post, I have been in touch with the owner of autorust.com and he said they are actively working on making their excellent Safe T Cap solutions available for 3rd Gen 4Runners.

So, at the worst, you bought yourself some time and you will have other more serious options available.

Check it out here. They already have kits available for 4th Gen 4Runners:

Toyota 4 Runner 2003-2009 Archives - autorust.com

I bought a kit from eBay but I don't plan on installing it. It seems like this is a common problem for Jeeps and they have many of these types kits available. It would be awesome have the same for our 4Runners.

Thanks for sharing and good luck! BTW, your solution appears to be a pretty good design.