As "mudbug" says, it really depends . . . I had an '82 and had the same problem. I also had an '84 with rusted floor pans. The '84 was worse, so I consulted a friend who is a body man and we essentially did the needed repairs with fiberglass on top and undercoating on bottom. That held for years, and when my daughter totaled the car, the floors were still just fine. As for the '82, it was just a spot rusted through, so I cleaned it up as well as I could, and epoxy'd a piece of sheet aluminum over the spot, and then applied under coating from below. That worked just fine, too. Of course, those were the days before POR-15, so you might want to consider that, though I personlly have no experience with the product.
The repairs I mention are not "permanent," but I live in the South, so I knew the repairs would last as long as I needed them to. Your situation may be different. At any rate, just take out your seats and carpet, and get a good look at what you have. My experience is that the rust throughs are generally small and isolated, and those areas can be repaired without too big a deal.
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