Im definitely the type that makes a money pit out of a vehicle I really like. But if there's one thing that's absolutely critical to a vehicle, it's the frame. If you're looking at replacing the frame ontop of all the other work needed to restore it, you're better off cutting your losses and selling the vehicle in this crazy market. I mean, as of this week, they really aren't making new pickup trucks.
Unless you absolutely love this exact truck, or are stuck with it, then swapping the frame off the bat just doesn't make sense imo. The rest of the truck would have to be absolutely perfect. There's nothing saying that $5k is going to cover additional damage that they find. If the body is rusted to match the frame, then you're going to wind up with a mess once you start to pull it off the frame. Once you do that, there's no going back. You either follow though or scrap it. And seeing as we already know it needs more work than the frame mechanically.. yeah..
Your call ultimately. I'd weigh how much you can get out of selling it (if you can) vs the cost of a good alternative one on the market. If it makes sense to you, go for it. My concern is that if they're going to wind up pulling the frame from another truck, id be curious what that truck was like first. You know? Rust is usually what kills these things. It's uncommon for them to be mechanically totaled. It's usually rust or collision and you want neither of those. So if they're pulling from a good used truck, id be having a really good look first to make sure you wouldn't be better off going with it's replacement.
The exception would be trucks that were caught in a flood. Those are the ones to get off of auction because they would be mechanically totalled and the interior is likely bad. But the steel should be fine.