351-C valve breakage has to do with metallurgy and that is not visually ID-able. The safest thing is to replace the stock valves with one-piece valves. Being one piece, they will NOT be 45-yr old OEM (which is good) and will not have been welded. Warning: cheap stainless steel valves can also be made by welding a head and stem together, and whether the cheap alloy or the welding is to blame for breakage is irrelevant. YOU are the one that has to sweep up your engine and start over. The offshore valve makers will not care either. Good luck- you've been warned.