Driveshaft length ??

So I take my 70 duster and drop the 7 1/4 rearend and put in a 8 1/4 out of a 74 dart. I go to put the driveshaft in, and remembered that the 8 1/4 is larger, so I know it won't fit. I drop the car back down put it in the back of the garage and start hunting for a driveshaft for a 904 trans and 8.25 rearend. I get a FABO guy that says he has a 904/8.25 driveshaft out of a 73 duster, so he sells it to me. I get the driveshaft and it is 3 inches longer than my original driveshaft (which is already 1.25 inches too long). Now he won't take the driveshaft back. My question is, that either this guy is jagging me, or I am missing something, which is it? I have the dart driveshaft, and it is even longer than the one this guy sold me, hence the longer wheel base. Just need some clarification on this. Any help would be great. :protest: