I think this is the driveshaft you are looking for. With a tape measure it measures just a bit over 42.5 inches center to center on the U joints. Probably 1 tenth of an inch. It is the black one in the pictures. I have a tape measure with marking on the back to allow you to wrap it around a round object and measure diameter. It shows it slightly less than 3 1/2 inches in diameter at the thicker part. I removed this from my second 'Cuda, which was a genuine 1970 440-6, hemi 4-speed, super track pack dana 60 equipped car. Unfortunately, when I got it the 440 was gone and a 383 was in it's place. The shaker was bondoed to the bottom of the hood and the car was so rusty that it was literally falling apart. These days you could save it. In 1982 that was a different story. Even the transmission had issues and I ended up finding a different one that I still have to this day. I am about to put Gear Vendors overdrive behind it. But I digress...
The red one is the 8 3/4 shaft I was referring to. It is actually from a Challenger now that I think about it. I know they have a longer wheelbase but don't know how that that would exactly affect the length. It appears to be about 1.5 inches (or slightly more) longer than the black one. When I sold the dana from the six pack car above I got an 8 3/4 rear axle from a friend that he had gotten from a 1970 (I think) Challenger. He also gave me the driveshaft, shown. I had them in my 'Cuda for a long time but never had it on the road. It seemed to fit ok, but I never drove it with that shaft in it. It will be back on the road soon. Since you seem to know what the various applications were lengthwise, I thought I would ask if this sounds correct for the Challenger I got it from or what it seems to be from. I will either sell it or have it shortened for the Gear Vendors...
Are you interested in the 'Cuda diveshaft? It currently has the slip yoke installed and u-joints on both ends. It has probably not been in use for close to 20 years and has been in inside storage with me that whole time. I am in Minnesota, and expect shipping could be spendy because of the size and weight. No idea what it would be though. How does 150.00 plus shipping sound? That may actually cover the driveshaft work I need to fit my overdrive... I would pack it carefully to ensure no damage in transit.
Thanks.
Tom