I take it you have the timing cover off then?
It sounds like you are putting a 351w dist. into a 302 block. Also, I think there were a couple of different lengths of 302 dist.
At this point it would probably be easiest to go to a good parts store (NAPA is the only one I can think of for this), and explain what the problem is. NAPA is usually more willing to pull parts off the shelf and compare to what you have. If Schucks can't find it on their computer it doesn't exist. (Sorry for the rant).
Wish you luck.
Come back and let us know how it works out.
jason