Is this the correct U joint to convert a 1LE shaft with external clips to an 84 rear end with the factory yoke and internal clips?

https://www.summitracing.com/parts/mog-372

Yeah, probably maybe.

They give you dimensions on the summit part info page. If those don't reassure you it's the correct one for your application, look up dimension description for 1310, 1330, 1350, and 3R u-joints. Then measure what you have.

It worked, ended up getting it at advance local for $20