Threaded fork query, steerer tube length etc.

If it's french and from the '70s it might be weird.

Vernier calipers are cheap and a good tool to have.

Steerer length is head tube length + stack height of headset (+ stack height of stem & spacers for ahead)

For 1" threaded headsets, stack height is typically about 30-35mm, and you can insert a spacer between the upper cup and the locknut to take up a small amount of slack if you don't fancy cutting 3-5mm off your steerer to make it fit exactly.

Note that the Park guide above has not been updated to reflect current science, and threadless steerers should go all the way through the stem clamp with a small spacer between the cap and the stem.

The cup fit is the same on French frames as on proper ones, so you'll be able to fit a modern fork and eliminate any French exceptionalism which applied to steerer threads, crown races and bores.

Stupid question or not? Was changing my quill stem and noticed that headsets top nut has about two-three threads left free not on the forks steerer... As a big bike mechanic got thinking that my steerer was cut to short and not exactly with headset top nut.. Should i change spacer to smaller one so i can tighten the headsets nut all the way so no threads left.. Or this is how it should be and its all good?
Thank you guys for help!

If the forks are the original ones that came with the frame then there is no reason the steerer would be too short. Having said that, the thickness of the spacer should be just enough to allow the top nut to screw on firmly without touching the top edge of the steerer.

Stupid question or not? Was changing my quill stem and noticed that headsets top nut has about two-three threads left free not on the forks steerer... As a big bike mechanic got thinking that my steerer was cut to short and not exactly with headset top nut.. Should i change spacer to smaller one so i can tighten the headsets nut all the way so no threads left.. Or this is how it should be and its all good?
Thank you guys for help!

As yoav said, you don't want the lip on the top of the locknut to touch the steerer otherwise the locknut won't be applying pressure to the crown race to keep it in place. The Barnett manual suggests that there should be at least 4.5 mm of thread for the locknut to get onto and that you should check that it's not touchin the top of the steerer: