351w

I worked at a parts store doing shopwork from '70 to '73. Back then Ford did some kind of cost reduction in the metallurgy of the heads that caused lots of problems.

The local Ford dealer started sending us 351W heads on a regular basis that had as much as a quarter inch of play in the valve guide and the valve seat all wallowed out. These were heads with 15 to 20,000 miles on them. These were 71 or 72 heads. Usually just one or two valves would be bad, not all of them.

The dealer had a pile of bad heads he sold us cheap, we repaired the guides and seats so we had a fresh head ready to go in exchange the next time a bad one came in the back door.

So if you have some early 70s 351W heads look closely at them. They might have been repaired somewhere in the past.