Sounds like the right approach.

If it is a new sender, it is most likely an 8 bar unit. The dash instrument will never go to full scale as the pressure relief valve opens at 6 bar. If you managed to buy a new 6 bar unit let me please know where.

The series 1 relief valve consists of many parts. The part you show is the series 2 relief valve. This one is known to very very occasionally not close. On the picture it is closed and looks properly cleaned. I doubt that valve caused the problem but it is not 100% ruled out. If you can find a serie...

Why not use the Midlock tester with the blue liquid? Hold the thing over the radiator. Squeeze the rubber ball several times. If the liquid turns yellow there is exhaust gas in the coolant.

I agree that head gasket is most likely. What is a leak down test?

Did you check the radiator temperature if it is equally hot everywhere? If it is the original radiator it may have blocked channels.

There is no problem if you run the car with DHLB32's. You just will have a few HP less power at 6000 rpm. That is if the combination of venturi and jets is a valid one. If not valid you may harm the engine. The DHLB35's have venturi's which can be changed. For the 1.3S engine you need the 31mm ventu...

That is a smart idea to fix the weights to set fixed advance. Then check dynamic advance from there.

Perhaps take the springs off for checking the max advance at high rpm? Mmmm. What do you think?

Over the years I have become critical about the max advance to protect the center main bearing.

Good question. Often I am suspicious about the springs like you. In practice it is not necessarily bad. There are other things which may influence the curve such as play on the spindle, play on the pins holding the centrifugal weights etc. What we need is a low cost instrument to check the advance c...

The castellated nuts have to be locked with a cotter pin. Lancia uses cotter pins or self locking nuts when the torque is low. Without looking it up I would set them at 2.0 mkg.

Note there should be a metal bush inside too.

The throttle cable should have room for the engine to tilt without influencing the outer cable. It should also be flexible enough. The ones with the steel spiral are usually ok. Cables with only plastic as outer cable are not ok. The plastic is not flexible enough after some time. Where the cable go...