FYI ...
The problem which you describe will manifest itself if the fork is
DAMAGED ... the legs are probably skewed by what may be a barely perceptible amount ...
Unfortunately, the only remedy for the average person (i.e., a person who is not a FRAME BUILDER) is to buy a replacement fork ... while a damaged steel fork can be repaired, it is NOT easy to remedy ... and, it had better be a heck-of-a-collectible (i.e., "rare") fork for a person to bother to have it repaired.
Figure on a cost of $40+ if you DIY as far as the removal-and-replacement ...
I recommend that you DON'T get a fork intended for 27" wheels even if your bike has 27" wheels-and-tires unless you absolutely need to use a front fender.
While a threaded fork will allow you to use the bike's existing headset, it will require MORE (IMO) effort to trim the length of the steerer whose amount of threads you will need to be sure are present and then chase the threads where you make the cut ...
OR, you may have to have additional threads added to the steerer which will cost you anywhere from $5-to-$25 per inch depending on the shop.
Presuming that you are not a purist, a threadless fork + appropriate headset are the better option UNLESS you have a friend who saved the fork from a bike which was converted to a threadless fork way-back-when..