Ken, as you already know, 374251-S2, or 359359-S2 caged nut (1/4-20) is the piece you need . Discontinued from Ford but is a AMK manufactured product (B-12033). They sell a 2 piece bag for $4.00, but unfortunately they have a $30.00 minimum order. I would check with Don (OMS) or one of our on site venders that handle AMK products to see if that is a product they stock. The bolts if needed are 1/4-20x1/2".
You could also check with Don (OMS) or Mike @ Motor City Mustang to see if they might have some used ones since they both have parts cars. And as Eric suggested you could always take your sample to a local hardware store and they just might surprise you.