Have you tried using a pin stretcher to make the pin longer? They are to expensive to buy but maybe you can rent one.;):B
These pins are pretty much standard size and length and I would be surprised if you checked and didn't find an existing plate already there. Many people use these plates so it is likely that the prior owner left the old one on when he sold it. I just picked up a used 5th a few weeks ago and discovered the plate was already on it when I tried to put on the new one. I took off the old plate and put on the new one, it fit fine