I am going to take a crack at this from a different angle. Are his shafts regular thickness or HD ones that are thicker? We all saw Bill Connely spintron video years ago. As a MFG  of shafts I had access to a spintron  long ago but with a wedge head at a different angle than his. Viewed the shaft long ways. Stock shaft vs, spacers vs Hd shafts vs spacer. What a difference in the wave. Get a surf board LOL. But an idea. This solved alot and yes frequency changed/ resonance. Maybe what you guys think.

 Doug aka pumpbldr
Precision Oil Pumps

Doug - Shaft bending is certainly a possible contributor.  There could be just the right combination of soldiers marching in step on this bridge.  I have a feeling this is going to be a really weird combination of factors.  As Brent says, a steel rocker should have no issues at typical cammer spring loads.

Very nice Brent!  That's a solid torque number as well 

Kevin - The pictures show a block casting number C7AE-A.

Fordification's site shows this is likely a '68 Marine block.

https://www.fordification.com/tech/castingnumbers-FEblock.htm

Oh man that is so cool!  What a great story- reuniting with your old carbs thanks to a thread on here.  I've always had a soft spot for those inline Autolites, ever since I saw some at Carl Holbrook's shop back in the day.

No it is a genuin 289 Ex racing cobra, thats why the wheel flairs and other modifications

That's awesome!  The 289 does sound quite spicy.  When I was a kid we had a neighbor who had a yellow 18 foot 1969 Donzi speed boat.  It was powered by a single Holman-Moody 289 HiPo engine.  Man that thing would rip!  He bent his prop one day and when he called the local marina for a replacement, they looked in the books and found two props that would fit that boat.  One had the bigger pitch for a big block, and the other was the smaller pitch for the small block.  The number on Jack's prop was the big block one!  The guy at the marina couldn't believe that little 289 could swing such a big prop 

I love it!!  Up until a few years ago Boeing had a service facility for B-1 bombers close to my house.  My neighbor worked for Boeing and would let me know when one was going to take off.

The sound was unbelievable because they would take off on full afterburner.  If it was evening you were treated to the four columns of blue flame with shock diamonds inside.  That's freedom!!

Those early 4.6 2V's had some valve seat wear issues.  You've probably sunken the valve into the seat enough to use up all of the hydraulic adjuster travel.  Your fix should work for awhile, but the valve will continue to sink.  There are probably some others awaiting the same fate on your engine.

Where did you take the material off the hydraulic lash adjuster?  Hopefully it was off the bottom.  Also, that oil hole on the finger follower is important.  It allows bleed oil from the hydraulic lash adjuster to flow out and splash lubricate the roller and cam lobe.

While I never met Tom in person, I enjoyed reading his posts.  He was a very knowledgeable and passionate FE guy.  You will be missed Tom!