Here are some pictures of a '79 (no belt attached here) and a '85 (attached) seatbelt upper mount:
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Both cars have some trim removed, so don't let that confuse things. Neither of these have the little "button" that your's does, but I think the mounting point here is higher than the button on yours. To help get some perspective, the 'slits' in the upholstery (both cars here) is where that little plastic vent goes (same as yours).
Here is your pic again for comparison:
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I think your button is more where the green arrow is here (or even lower):
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