66 Dart u joint

I'm having a similar problem replacing my 7260 ujoints. I bought the Spicer 789 solid ujoints that are supposed to be the heavy duty replacement for the Spicer 1306. Well, I cant get them installed. Here's why. You cant get them to go in so you can place the caps. The top pic is the 1306 joint it slips in easily. The middle pic is the 789, it wont.