Is the fork for a 20" or 26" bike? 26" forks usually have a very long steering tube that needs to be cut down. A 20" fork should fit most 20" bikes, with the exception of some oddball bikes or girls bikes which can have very long steer tubes. Even if you did cut and reweld, it would likely change the geometry of the fork to the point that the spring-bolt bracket would be difficult to get the bolt to thread into once installed and also keep the bracket from sitting properly onto the steer tube if you did get the bolt in. Some pics of what you are working with (the frame headtube as well as the fork assembly) would be helpful.