Seems that I read a similar topic recently wherein an air lock in the cooling system was discussed. Usually it's in the heater core circuit. How well is the heater working?
OH BUT THIS: Is the radiator specified for a 1968 IH gas powered truck or a 6.9/7.3 IDI diesel? Curious: is that a pick-up or an old Travelall (spelling?) I remember those, 345 cid v8 and I had a box truck, (R-160 series bread truck, box separate from the cab but the back of the cab and front of the box were cut out for walk through - had bucket seats.) with a straight 6, completely rebuilt, and I liked that engine a lot! I think it was a 1959 or 60, maybe 63. I feel SO SAD about losing that truck and how I lost it and this is 52 years later!
The pressure relief cap on the IDI diesel radiator is 13 PSI.
Aside from other suggestions, also be reminded that we can't *SEE* your cooling system set up and you mention "electric fans." One good photo can help quite a bit for us to *see* what might be going on there.
I might suspect that the 'electric fans' are not moving enough volume of air. They might turn fast and push or pull with some velocity but not enough volume. The stock clutch fan has huge blades, has a tightly fitting shroud and moves a huge volume of air through the radiator. I've never heard anyone complain about a 6.9/7.3 specified 3 core aluminum radiator not exchanging heat well enough and it's a common replacement radiator.
Aluminum radiators are supposed to be very efficient radiators in terms of heat exchange. IS THIS AN AUTOMATIC OR MANUAL TRANSMISSION vehicle? (and if it's manual transmission you have a radiator for a manual transmission and not for an automatic?)
Where are the electric fans mounted (In front of or behind radiator?) and how big are they? From what kind of vehicle, or aftermarket? A PHOTO MIGHT HELP. You also need an adequate fan shroud for fan(s) to work efficiently, otherwise the fan(s) will pull air in from the sides (least resistance) instead of through the radiator, or push air out the sides if in front of radiator. There is a loss of efficiency without a good fan shroud and for 2 electric fans I think that ideally you would need one shroud that covers the whole radiator and fans that pull A LOT OF AIR.
Those are my thoughts. Sounds like an interesting project.