Seat bracket bolts?

[cswagon1]

My '69 van had a pair of Ford Ka seats fitted when I bought it a few weeks ago but I'd like to revert back to standard.
Unfortunately , the bolt holes in the floor are pretty gummed up and I don't want to try and clear them with the wrong
size tap and mess them up.
Therefore, would anyone know what size they should be please as I've found all sorts on this one so far. UNF, BSF and
a couple of metrics which I presume was a P.O "upgrade".

Many thanks

Graham.

[les]

Thread size —— 5/16 BSF.

[cswagon1]

Thank you Les, that's very kind of you.

Graham.