After letting the stuck seatbelt retainer bolt soak in PB Blaster for a few days, I was able to remove it this afternoon, while the kids were napping. After I got it out, I ran the shop vac hose into the space behind the seatbelt retainer. 37 years of accumulated oily dirt and crud can really choke a vacuum hose.
Once done, I celebrated by mowing the front yard. PAR-TEE!