Acarlan

Hey everyone, obviously I'm new here but I'm stuck and hoping that there's someone on here that could give some guidance. I have a 2004 Express 3500 that recently had a transmission failure. After receiving a quote from the local shop to replace it I decided to look around for another compatible transmission. I found one for a super good price and based on everything that I couldn't find it seemed to be exactly what I needed. The vehicle it was out of was a totaled (tree fell on it) '99 c3500 with the 5.7. It was a remanufactured transmission with only 70,000 miles on it and had been fully updated to the newer specs when it was replaced in 2003.

All was looking positive until I took it to the shop this morning. The guys there said it wouldn't work because the output shaft was different. Based on everything that I can find both the 99 c3500 and the 2004 Express 3500 have a full spline shaft with a slip in yolk. I'm guessing there is a difference in length? Is there anyone on here that knows what differences are? Any help would be greatly appreciated!