Hello,
ok, I think the time to work on the engine is arriving.
I will proceed as follow, first I will remove the intake manifold for two reasons: one because for a lot of time the car was running very hot, I have alreadt replaced the radiator but I think the reason is also the little radiator under the dash, and there are signs of steam from intake cooling circuit, but the principal is that I can observe the famous lifter, push rod and rocker arm of intake cil. nr.7 during all the movement, trying to understand and found some issues.
Then I will replace the studs rocker arms and I will proceed to reinstall a new rocker arms, before of this I will put prussian blue onto the stud and carefully re assemble everything. If interference accour I will found the rocker arm contaminated with the prussian blue.
An other way to check interference is to keep an old rocker arm and enlarge the slot and check if there are difference in the complete lifter height of the rocker. This is hard to understand if the difference is minimum. I know that even a 1/10 mm (0.004") is enought to cause a broken when as an hammer the rocker received the blow.
Than I would remove the spring so I can verify if the valve properly work.
So it is all a research phase but it is important to understand ...at the third time which has broken.
Marco.

6sally6 wrote:

You could always by-pass the heater until you get the rocker arm issue fixed....then come back to heater issue.
MAKE SURE you use the correct intake manifold gasket for you engine/intake. 289 and 5.0 heads are different (I believe) as far as gaskets go.
I like to use long bolts with heads ground off (4)to help align intake when setting on engine. Don't use the rear gasket (along valley cover) use RTV instead . Hard to get a good seal in that area.
Let us know how it goes!!
6sal6

Not just the 5.0.  The heads changed sometime in the mid '70s.  Most gaskets I find do not work with the old style heads.  The coolant passage holes are completely wrong for the early engines.  I used a set of Mahle gaskets when installing my Edlebrock intake that worked very well and had the right size openings.  MS15172 from rockauto.com. 

Hello,
the work started but I was unable to check the interference between the rocker arm and stud because the hydraulic lifter, not being preloaded by the oil pressure, absorbs the thrust of the spring and the rocker arm does not make the whole stroke.
So to simulate the preloaded lifter I thought to disassemble one that I have in stock and place a thickness instead of the spring to make it rigid. This could allow me to complete the verification. I'll let you know

MS wrote:

Why do you think you need a long slot rocker?.  This is a Ford, not a Chevy.  Unless you are using some ridiculously high lift cam, any of the 3/8" stud mounted rockers will work on your 289.  I have used the Comp Cams roller tip rockers on several engines and have use the cheaper Elgin (copy of comp) roller tip rockers on a couple.  Never any issues.

 
X-2!!^^^^^^^^^
6sal6