Looks like the only V-8 engine options for light duty trucks was 2 versions of the 352. IF it is the original engine, the 4th digit in the VIN would be a "Y" for the 352 or an "8" for a low-compression version of the 352.
Thank you, Sir! Glad it helped. There is a mistake in the '78 manual for the Neutral Safety Switch. The'78 manual shows the NSS attached to the column, as it was in '77 and earlier. That is not the case. On '78 models and later, the NSS is attached to the side of the transmission.
https://www.partrequest.com/catalog/air-conditioning-and-heat/ford/ford-f-250/1979This might be what you need? They are used but condition is stated in specs.
Just wanted to add that the earlier trucks with 11 digit VIN's have the 5th digit showing where the vehicle was manufactured. For example the 5th digit on my '78 VIN is "R", indicating San Jose as the manufacturing plant.
I like the color. My boss just got one for a work vehicle. He puts about 100,000 miles on a car in less than 2 years and has been happy with the explorers.