Yes, a 7.3 IDI will drop right into the 6.9's old home. If you have an '86 6.9, you will either have to get the "newer" 7.3 IDI glow plug harness, or you can remove the 7.3 glow plugs and put in new Beru/Motorcraft 6.9 glow plugs and use the 6.9 system. This is what I have done with my project truck. Now, putting in a 7.3 powerstroke is a whole other story. Sure you could do it, but it will be a huge electrical nightmare, plus the IDI's and PSD's are different breeds and will require more work to get PSD into your '86. What tranny are you going to use?