I'm working on the same area of mine, been trying to figure it out for a good two weeks now - it's a bad design, nice little cove underneath where water can hide that the regular P/U's don't have.

I'm just going to cut out what I can, blast the poo out of the panels from behind and underneath then weld in what I can, where I can. Seal it then have the floor sprayed with bedliner. I'm considering re-locating the seatbelt attachment point to a place above that panel and behind the pillar too - at least theres some structure there and you can get to it.

For what it's worth, that part is spotwelded and "sealed" all the way around save those two bolts. If you've got rust on that structure piece behind that panel, be sure to fix it best you can too.

Good luck man!

 

my first 'Burb had this issue as well.

i was giving my wife S**t for banging her boots off in the Winter, and getting slush 'n crap all over the inside of the windshield and dash pad. i was always cleaning the inside of the passenger's side of the windshield & dash pad area, and of course she denied it was her.

i figured out where the mess was coming from the second Winter i had it, when i was backing up in deep snow, my son was laughing his *** off! i looked in the mirror @ him, asking him what he was laughing @, he just pointed to his little sister sitting in the passenger seat.

i looked over, just in time to see chunks of snow flying through the air-landing on the dash pad & windshield-and my daughter covered in snow! turns out i had a 3" hole in the panel that dropped down from the rear cargo floor, to the floor pan for the rear seats. it was right up against the side panel, and of course the carpet didn't cover this, so everytime i reversed, it spit s**t all over the inside of the truck.

i of course apologized to my wife-who gave me that look, that we all know

 

This is a very common rust area. The good thing is that there are no complex bends in that panel so it should be easy to bend up out of sheetmetal and just welded in.

Now the reinforcement behind.... That is a different story.

 

Yes, that's my worry as it is a crucial area(seatbelt attachment). I'm thinking of making a box channel that welds to the inner and outer skins just to be safe, and also using 16-14ga steel for the inner panel. If I remember right the bodies in these years were made from 18ga from the US and Canada, but now allot of the metal we get is from China and is of substandard quality. I was bending some 1-1/4 pipe from China a couple years ago and it was cracking at about 50 degrees, so much for the days of 180 degree bends.
I also plan on smothering the area(and the rest of the truck for that matter) with some sort of undercoating. When I get around to doing it I will also draw up a template of the channel and panel to upload here for other unfortunate souls
.

 

I have a feeling the seatbelt attachment point is the reason the regular suppliers don't (won't?) carry a replacement panel. The liability would be substantial with DIY-ers. I don't understand why GM put it at a weak point though, the belt hardware wasn't attached in any way to the structure piece before the rust took it out. Go figure.

 

I'll have to snap some pictures of this area when I strip the outer sheetmetal off the donor fenders I have. They were off of a mid-80's K5.

 

I got (most of) the piece out of mine today. There's more to the panel than meets the eye. Appears to be more part of the fender than the floor. Snapped some shots.







I'll snap some more tomorrow once I blast the remaining rust out.

 

Hey Trusty did you every get you b-pillars fixed?

 

Not yet, I have been fixing more important things (in my opinion) like tie rods, drag link, stabilizer, (had a BAD death wobble) and a gaping hole in the roof @ the drivers door, which turned out to be more of a job than I expected. I started grinding/cutting the rusted area and found a sh** load of bondo. Looks like a tree fell on the roof at some point and instead of trying to pop the dent out they just bondo'd the heck out of it (directly over the paint). Now that I have the more important things taken care of I am a little short on cash for panels.

 

Tell me about it. Did both sides of mine and all the other minor holes, turned into a top-to-bottom interior redo with about a grand worth of new panels and stuff. BUT - Very pleased with the results.

I'm fixing to chop the entire roof section off of mine from the B pillars to the windshield frame - Bought a '74 with a good hard top and I'm going to convert my '86 to a full-convertible top. Shame you're not anywhere around here, I'd hook you up with an entire roof plus a freshly painted fiberglass top.

Good luck man!