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Try Fastenal ? Do you have an old one to show them? They can most likely get you a box of them in a few days?
1986 300 ford motor exhaust manifold bolts - Search (bing.com)
I wouldn't fear using the old hardened fasteners if you use blue Loctite on the fastener threads. Wire buff them to clean up. Shy away from stainless, they break.
Paint the heads once you have it all torqued down. If you can't find exactly what you are looking for use hardened steel flat washers.
Impact sockets are best where you are, they protect the head of the Capscrews.
Just drive them on. A run to Harbor Freight is a wise and inexpensive way to go for tools.