For a 1974 390-out of a pickup, there is only one standard Melling pump shown, (the M-57) The only other listing they showing the 390 that year is the P-350 step van and its showing the same pump number.
Here's a link to the M-57 to check specs against what you have.
In looking up the 390 pump it reminded me that it was one where the oil was biased to the top end enough that it was considered to be overoiled there. I don't remember if Ford ever fixed that or not. Some people, especially those who wanted to run a high volume pump used to put in gallery restrictors to reduce that tendency and also do other things to improve bearing oiling. The restrictors, being smaller diameter than the galleries, are an even better place for stoppages if there are pieces of gasket material or other crud getting circulated.
On a related note, you just mentioned the oil pressure as originally having dropped way down.
If that happened fairly rapidly, besides it being posssible that the relief valve was stuck OPEN, its also possible that the old pump's pressure relief valve was stuck
CLOSED.
If THAT happens its possible to raise the oil pressure high enough to force out one or more oil gallery plugs if they are the
pressed-in-type.
If that did happen, the oil pressure will be "way down" from then on, making it difficult to force oil up into the top of the engine for ANY oil pump.
And especially difficult when turning the pump by hand.