Greetings A Body Members
You can do a couple things here, we offer joints that are 3/4 smooth that you can weld to your shaft or joints that are 3/4"DD that would require you to grind two flats on the shaft to turn it into a DD profile. Your manual steering box is a 3/4"-36 spline. A joint 3/4"-36 X 3/4" smooth is our part #013464 a joint 3/4"DD X 3/4"-36 spline is our part #013449. I have attached an example of a 3/4"DD profile if you are unfamiliar. It is basically a 3/4" round shaft with two opposing flats .550" apart.