1971 198 slant 6 manifolds

The bolts are standard hardware fare, there is nothing special about them. Allthread would work fine too. The single bolt closest to the block is longer so best to measure before buying. There is a gasket that goes between them, it seals up the exhaust that comes up to heat the intake. Without it you will get blow-by all over. That gasket is nothing special, you could cut one out of gasket sheet material. I would bolt the two manifolds together first making sure they line up with each other for a good seal to the head, then install them as a unit.