Good Luck. Just had a similar problem with my 428. Periodic lifter noise got worse on number 5. I too thought it was just a collpsed lifter. When we got into into it there were 4 mushroomed lifters and 4 rounded lobes. Melling camshaft. Engine run-in by the builder and also run-in by me when the car was assembled.
Oil changed 4 times in 2800 miles. Fortunately, when I decided to fix what I thought was a collpsed lifter, I also decided the car "needed a little more cam" so I dropped by to see a friend of mine (local FE expert, has a machine shop and also an original GT-40) and he put me on to Chase at Crane Cams. Chase speced a road race grind for my engine and drive train. With new lifters about $300 Gazoutas. (Goes outa my pocket). Car is now back together. Just put 100 miles on the new cam and just changed the
oil. As a side note, the car is just a bit louder with the new cam and I just moved into a new town house. Neighbor upstairs came down when I fired it up in the garage and I thought she was going to complain. She almost started to cry. Seems her high school flame had a "Real One" and it brought back a few memories.