[Dead Parrot]

2nd the penetrating oil approach. Crawl under and spray the threads from the bottom. At least twice a day. For several days. Penetrating oil isn't an instant solution. It can take days to do its thing.

Use a pipe wrench if the seatbelt stuff allows. You can grind the marks off once you get the bolts out.

Another option on the hit it with a hammer would be to use some scrap 1/2 iron pipe or other sacrificial thing as the interface between the bolt and hammer.