Roger - you need some reinforcing for those seats. Fender washers are not sufficient to hold a bucket seat through a fiberglass floorboard. I trashed the Brookville supplied soft, draw quality 22ga steel floorboards after kneeling on them and having them bend under my 200# weight. I had a double 16ga floorboard made up and then welded up some .500 square tubing and put that in the middle - i.e. meat in the sandwich along with some insulation. I added some tabs on the tubing web then through bolted the seats. These floorboards are in turn bolted via many Gr5 .25 bolts to the body sub frame. I would, in your case, probably make up a steel tubing web, bolt to the underside of your floorboards then bolt your seats through that. Is it necessary to fasten to the chassis? Probably not. Additionally, you could allow for your seat belts to bolt to that seat frame too - which I did as well.

For whatever reason, I don't seem to have any pictures of the assembly - probably because I was 'between cameras' when I was doing this part.

Hope you can figure out what I said If not let me know if interested