Cut the heads off of two or three old bolts to use as a guide for the new manifolds - once you have the old manifold off and cleaned up the surface, loosely thread them into the block, hang the new gasket in place over them, put the new manifold in place, then after you've got 3 or 4 new bolts in remove the headless bolts and continue.
Trying to have someone hold a 50 lb manifold in place while you're trying to line up the gasket and thread a new bolt in is really hard.