Well it certainly appears that the one you bought has the correct length but the cam plate and weights are special.

IMO it's definately worth asking for pictures of known correct parts. What seems most odd is the weights made of built up steel plates - the part book indicates it should have the same weights as most other distributors. That's what makes it appear to be a special, low production item. And if it is special but for some other application it would suck to go through all the trouble of drilling and installing it when a) it may be valuable to someone else b) you could get a fairly correct advance curve using a shaft from a good used 383 distributor. Since you have a little time, why not see what knowledge and pictures are out there.