Shaft Length

A) Make sure your axles fit the splines in the diff. before doing anything else!
B) If you are using the "green" bearings, then make sure the thrust button is not in the differential. You don't need it and don't want it. If you can't more or less see straight thru the splined axle holes in the diff., the button is still in there. Sometimes they fall out, but you'll probably need to knock it out with a punch and a hammer.
C) Assemble the axles, bearings, and backing plates to the housing without the carrier ("pumpkin") being installed. If the two axles hit each other in the middle, you need to cut them both off enough so they don't hit. If they are not hitting each other, you should be good to go. Unless they are too short and won't engage the splines in the diff. which should be easy enough to see. Not likely though.