You have your work cut out for you. The hard part is the electrical. If you have the complete donor PSD it's definitely doable, but it'll be a project. The nice thing is that all the Ford diesels use the same bellhousing and bolt pattern. What transmission does the 88 have? I'm guessing the PSD donor has an E4OD automatic since you said ambulance E350. Most stuff is common between the C6 3 speed auto and the E4OD, I think the torque converters are a little different, and of course you have a computer for the E4OD and just a VRV on the C6. I don't know if anyone has tried to mate a C6 to a PSD but man that would be interesting trying to get them to communicate. If you have the manual ZF5 then it would be easier, but then of course you need a clutch, pressure plate and flywheel for a diesel. I'm not sure if all the clutch stuff is common to the 87 and later IDIs and the PSD or not. If you just have a donor motor and not the whole vehicle it probably would be too hard.