universal joint fitment issue

I've rebuilt a 8 3/4 rear that I'm installing in my 69 valiant. It isn't the rear it came with (someone previously transplanted a 340 and 8 3/4). The first problem I encountered was the U joint in the propeller/drive shaft mating to the rear. The U joint on the drive shaft yoke was way to small to fit the pinion. I solved that problem, sort of, I found a thread on conversion universal joints and sourced one out. I've pressed the old one out, and the new one in, but now the problem is that the only one of the cups will seat in correctly. It seems the c clip that fit the cups are too wide to fit the channels in the pinion.... weird right? Am I missing a crutial step, should I be putting the c clips in after the joint is in and the straps are bolted up? Can I install without the c clips, bolt the retainers, install one c clip, then gently tap the other side until I can install the second? Cant find any videos or step by step instructions. Any help would be appreciated.