If you ever get that car near Gainesville I would love to see it. I have never seen one.
I suspect the manifold pressure sensor from a 3.5 would be good enough. The 3.5 had the full load stop built into the sensor as I imagine the 250CE did. The 4.5s had a five wire throttle switch and electrically delt with full throttle mixture. Check the throttle switch, if its four wire the 3.5 sensor will work.
I also have never seen that distributor, but the 164E Volvo would be very similar. The distributors might work. I presume that the 250CE has two point trigger points like the Volvo.
Bosch Master, ASE Master, L1
33 years MB technician