It would help if I could get a definite on the actual OEM part number -I have seen 3 different ones, and then when I go to buy them it pops up as incompatible.
I found the right part number, I think: E8UZ-9A758-D
This page lists it as being for 87-89 E350 with a 6.9. I don't see why it fit a 91 with a 7.3? Same (basic) engine, same body. Unless they changed the throttle pedal after 89?
https://www.fordpartsgiant.com/parts/ford-cable-assy-throttle-control_e8uz-9a758-d.html
Here is one on ebay, but it's $89.
https://www.ebay.com/itm/throttle-c...440995?hash=item1a7b0a95e3:g:RQIAAOSwK6RZLwuT
The RockAuto listing of the Pioneer cable for a 91 E350 7.3 says this:
Alternate/OEM Part Number(s): E4UZ9A758A, E7UZ9A758E, E8UZ9A758A, E8UZ9A758D
The problem with OEM part numbers is they sometimes change a lot over the span of 25-35 years. So really, all of those are the OEM part number, in theory anyway. If you cross reference the first number I cited above (E8UZ-9A758-D) you will find this:
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Source: http://www.kakapart.com/oem/partscrossinterchange/ford-e8uz9a758a.html