I seriously doubt the shear pin issue was caused by the extension--the trouble we had was with the high moisture corn we had last fall guys would let the corn pile up to the boot and the grain had nowhere togo so it would roll back the end flighting--mostly operator error!
Try Niels Parts, I have a stewart steel ext on my A85 and it seems to work well. I needed it for a 45ft macdon. I did shear the spline on yoke and shaft between the cross auger and out load auger and shear bolts but this wasn't due to extension. A bit of welding and it saw 85% of the harvest out. Another fella with same set up had no problems. Only thing I dont like is the crappy spill flap of agcos on the end of the auger. Replaced the spring a couple of times and only get a day or two out of it. When auger is still full it will leak grain, how much it is hard to say but it is a pain having lines of wheat in your malt barley.