Paul,
Well, I don't have this at home with me, but I can help none the less. For the bunk placement, stick your head in the bilge and locate the outboard stringers. Measure the distance over to the keel. That is where your bunks need to be, directly under the outboard stringers.
Now, since we haven't built a 251 for a few years now, if I remeber correctly, the bow eye should be about 3' aft of the leading edge of the pulpit. That would put you at about 22' to the transom. I know you are trying to get your winch stand set up to where the prow snugs up against the bow roller, yet the transom is supported by the bunks completely.
Check with Magi-Tilt, LoadMaster, we have bought from them for years. They might have the exact dim.'s you are looking for...

Bob C