During my engine (327) rebuild, torqued my SB exh manifold bolts to 20-30 ft-lb per spec. Bolts are 3/8"-16, probably grade 5 material. Torque of 20-30 ft-lb will "preload" the bolt properly. Enough preload to keep the joint tight while not stretching the bolt to a point of permanent deformation. Periodic recheck of the torque isn't a bad idea either.
Also, the gasket must be taken into consideration as well. Using a high strength bolt and torquing to a higher value could impact gasket integrity. Obviously the original specs are for original gasket, bolts, etc and might be different for performance upgrade components.