If it were I, I would leave that gun alone and buy something different. Sentimental value alone is worth the leaving. I wished I had kept my Dads guns the way they were to pass along to my kids. I did mods and barrels and stocks and now they arent the same. The 257 is a great round and there must be a reason that they will not fit in the mag correctly. What length are you loading to? If you are seating to the lands, they most likely wont fit the mag. But check the overall length in the book and then check a factory load and see what the loaded difference is in lenth overall. If thats the difference, then most likely you either have to load them shorter or shoot single.
Im not the guru on this, so hopefully somone else will chime in and figure it out. But I would keep the gun and get another, in say a 30 cal to play with and have the 257 to take out and shoot for memories.
Ballistically Speaking, Im about a 300 FP with a BC of -1.