Hello my friends, this is an update.
I came to the conclusion that the valves were floating when the engine returned to idle, so I removed the rocker arms and it's pedestals and installed shims under them.
I added up to four 0.3mm shims and the engine run great, never missed a beat, but the valves made a bit of noise, so I removed one, for a total of 3 shims which is 0.9mm (0.035 in)
With this shims the engine runs quite well, not as smooth as with the 4 shims but the valves are quiet.
The problem is that this ford pedestals pair two valves so I can't adjust them individualy, I feel I should convert to a fully adjustable valvetrain.
What could be the cause of this? I doubt this engine had the head shaved, I talked to the previous owner who had it since new and he said he didn't touch the engine ever, I replaced the head gasket with a Victor unit, good brand name, I doubt they messed up, and I see the gasket has about the same thicknes as usual. Is ti possible for the lifters to break appart or loose the circlips or something else, that makes them stretch beyond their limits and keep my valves open?
BTW I installed a brand new Carb and electronic distribuitor, just to make sure this engine runs as economical and reliable as possible.
Please comment my friends.