I would get a few extra long alignment bolts and cut the heads off them. Screw a couple in and slide the gaskets on and then the manifolds. They will support the manifolds while you start the actual bolts
Be careful- - - -those end dowels that make a convenient place to put long headless guide bolts for manifold installation SHOULD NOT be replaced with bolts. The dowels are there to allow the exhaust manifold to expand and retract from heat changes. Bolting the ends down tight will result in a cracked exhaust manifold. I like to install short Allen head bolts with the heads ground down to the diameter of the dowels instead of fighting with the short headless dowels.
Jerry