A nice clean anchor system that would allow removal of the seat belt anchors would be to fab up a steel anchor plate held to the bottom of the wood floor with screws and with a threaded hole for an eyebolt screwed in from the top. Then if you wanted to remove them, just unscrew the eyebolt from the anchor plate.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.