the shaft length needed is determined by your wishbones, so as long as you have the original wishbones you can keep your original shafts. the 5 stud outer cvs swap straight over to 4 stud shafts since you have shortened one of them that could be why it keeps breaking? but if the original shaft was 448 then somehow you had a widetrack short shaft fitted? try a new mk2 golf 4 stud short shaft that should sort you out anyway