on my 82, the replacement fittings into the exhaust manifold were 1/4" pipe thread plugs, maleable iron. (black/dark grey in color)
The galvanized ones appear cleaner, but using galvanized in a cast iron header didn't seem like a good idea due to heat handling characteristics.
The 1/4" pipe thread plugs should be available at a plumbing supply, don't count on Home Depot to have them
The other direction, which I was told works too, is to cut off the metal tubing into the original fitting leaving enough tubing sticking out and then pinching it off real tight.... I didn't want sharp ends sticking out on each side of block.