Exhaust Manifold Gaskets

I am in the process of installing exhaust manifolds on my 360 (273) manifolds.
With the gasket kit is a plastic bag with 2 small gaskets. It says part
#90714 (3 slotted bolt holes)to the rear of the right-hand bank and to the front of the
left-hand bank. How do these fit and what are they for? Also with the fastener kit
all of the washers are the conical type. I understand the washers on both ends
were to be flat hardened (grade 8) washers. Thanks to all for your input.