Id go to Ford and see if they can get the bolts , or order aftermarket versions . If you use the "hardware" grade 8's be prepared for them to break often for you . Ive yet to see any hardware store true grade 8 bolts .
If you have a caterpillar dealer near you you can buy bolts intended for a cat machine for the manifolds . They are very tuff bolts and won't break . You'll just need the thread size (IE: 3/8 inch X 16) and the bolt lenght (IE: 4 inches) .. just make sure you verify the lenght needed and thread size/pitch and they can set you up .
Id hit up Ford first , the manufacturers make (or have made for them) much better quality bolts .