The key is getting the mating splined outer sleeves when you purchase your EPS. I used, or modified, almost every part of the EPS I purchased including the firewall seal. You can certainly modify the splined shaft to accept a double D joint but you wouldn't want a u-joint on the input side (steering wheel side) of the EPS, you just want to "hard-connect" it to your existing steering shaft unless you are going to configure some sort of tilt wheel arrangement. On the output side of the EPS you do want a u-joint and where and how you attach it will depend on if you are going to connect it to the existing steering box, like I did, or if you'll be connecting it to an IFS rack and pinion.
I don't think the EPS units are designed for a lot of load pushing and pulling thru their length, the splines are there for crash protection (collapsible columns)and for assembly so you may want to take that into consideration if you aren't planning on using the splines. Send me your e-mail address and I'll send you pics of how I went about it.
Dave
