There are special sholdered and treated bolts that mount all the seatbelt anchors into the car from the interior side. The heads are fairly large, I am going to guess from memory and say somewhere on the order of 3/4 inch.
You will have to pull the rear seat bottom cushion out for sure and it might be much easier to pull the front seats out too. Pulling the fronts will make both getting the rear seat out and access to the front seatbelt bolts much easier.
And I strongly recommend investing in a set of factory service manuals.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
