rust in bed of '93 k2500 suburban

I pulled up the carpet in the back of my suburban and found that while most of it was in pretty good shape, there was some rust that was allowing water in. There appear to be two points of concern:
There is a cross-member spot-welded to the underside of the bed which is right above and parallel to the rear axle. I believe this cross member is where the body bolts to the frame. The cross-member itself is a squared off U-shaped channel with flanges at each side for spot welding. While I don't know what it looks like on the underside, on the top I see that I have paint bubbling over the channel and a small rust-through in one spot (rust through the bed, but not into the cross member).
The second problem is located at the seam between the pan and the metal that forms the wheel well on the left rear wheel. There is a sealer between the two that has started to come loose in spots, and rust is forming in the crack below the sealer. The seam appears to be formed by both pieces (the pan and the wheel well metal) being bent down 90 degrees to form a flange which is spot welded together and then sealed with the sealer (silicone?).
I'm going to start cleaning everything up to get as much rust off as I can and repair what's necessary, but had a couple of questions:
- I can wire-brush somewhat into that seam, but I doubt that I'll be able to get all the way into the seam without actually disassembling (un-welding) body panels. Assuming that I'm leaving some rust in the crack, is there any product that can be used to prevent it from spreading?
- Is that cross-member channel something that I should remove (un- weld) and replace? If I don't, I'm sure rust will continue to exist in it and maybe attack the bed from below. The rust-out is a very small area.
- IF I go through the trouble of trying to replace this cross member, is this the same place that a body-lift would be installed? I'm asking as I was thinking of doing a small (2") lift at some point, and it would be easier to justify if I had to get in there anyway.
Mainly I'm looking for a way to effectively stop the rust from spreading. The underside of the truck has the usual layer of rust in all the spots you'd expect - though to me it seems like it's in pretty decent shape - no cancer except at that cross member to bed connection. I don't want to put a ton of money into this truck, but I really don't want to just put the carpet back and forget what I saw.
Thanks, --Jeff
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Por-15 does a great job on rust.

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