I am guessing original engine had seen better days?
Unless you can find the person who decided to make the swap your guess is as good as anyone else's.
That truck is 54 years old and most likely a number of parts have been replaced along the way, and many of them with pieces different than what the factory installed in '55.
If you are trying to create an accurate restoration getting all those parts right matters. If you just want a driver, getting a combination that works well together is what counts.
