Seals are well the rubber umbrella (or not) style rubber on valves' stems? I will order a set of those.
I learned that there is some that slide over the guide and are held in place. Seems to be more efficient but would require machining of the guide so I won't go there.

How do we check the guide clearance? I pulled the valve a bit and check for radial play and did not felt any play. Is that good?
I also ordered the same spring compressor you shown above.

I don't have any rotator on the valves. I don't think I will go with that. As I said the engine was running fine, will see just street use so I don't feel a huge need for those.

Shorter springs (I understand they are those you said were intended for rotators) are on the intake.
I have long (2.2") springs on 1.500" exhaust valves and shorter (1.950" springs on 2.020" intake vavles.
Does it makes sense?

 

That is correct
You pull the valve out about an inch and see how loose it is in the head
If it is relatively tight, you are good
Well, if you have no rotators, that is okay but all the springs need to be the same
The shorter springs are supposed to have the rotators
You are going to have inadequate seat pressure on the short spring valves without a rotator
You should be able to get some rotators at a junkyard cheap
I'll try to post a pic of an old head I have that has all the valves assembled correctly for you

 

I was not sure about the different spring lengths. Went to Facebook groups and did not get any clear reason as to why I have 2 springs lengths.
When I go to part dealers, never saw kits with 2 springs lengths.
Now the engine was running great as is/was !?
In any case, I am using OEM rockers. How could I determine springs specs ?
Do I need to change the locks or is it OK to keep them?
Also it looks like the spring pocket on #3 exhaust is deeper. (I guess they all have been machined!?). I will probably have to use a .035" shim there?

 

I thought they all had the rotators
Apparently just the trucks, and only after about 1970
Apparently after a year or two they installed the rotators on the bottom
There are way smarter folk than me around, and here is a forum page from another site
I have learned a ton from the tri 5 site for the 55-57 chevs
They helped me decipher which motor I have in mine presently

 

The ones with the rotators on the bottom, they machined the spring seat deeper apparently.

 

You just need to make sure you have decent spring seat pressure closed
It takes a tool to measure it but the specs are roughly
80 lbs at installed height of 1.70 inches
110-120 lbs maximum (solid lifter cams 140 lbs)

 

If someone has ground the valves, or seats, and the valve gets sucked into the head...
That raises the installed height of the valve spring retainer in the head
That is when you need a shim or two under the spring seat
To maintain the 1.70 inches
Good luck

 

I am trying to mesure the height available between the pocket and under the retainer and it's tricky and getting mad and largely unconsistent measures.
Does it exist a tool to measure this?

I am around 1.700" - 1.760" I am not too confident in my measures.
In any case if I can't do any better, is this acceptable?

 

Yes, there are a few checking tools
Summitt should have one. You should be good to reinstall