If the steerer tube seems to be short of the stack height, just make sure you have enough thread engagement so you do not end up stripping the top lock nut when you tighten it, or loosen and come off when you ride.
Ideally, you should have the steerer tube almost kissing the bottom of the inner flange on top of most locknuts which on most cases can be fine tuned with washers when your steerer tube is slightly longer than the internal stack height of the headset. but then usually, older bikes do not have their original headsets and the steerer tube was cut specifically to fit a certain headset stack height, thus sometimes ending up with steerer tube a bit shorter than a replacement headset's minimum stack height. If things are just loo to iffy, just be safe and get a different headset that can headset that will work with the steerer tube length best.

Chombi