My 1977 B100 has a broken exhaust manifold from PO. No biggie I thought. Found new one at auto parts store. Took old one off (after cussing the exhaust flange bolts) and went to put new one only to realize the new one has AIR channel through it. i.e. a channel w/ an threaded open end on each end of the manifold which also ports to just right about the openings in the manifold. My existing manfold had dimples in the cast iron where those ports should be but was still solid.