So which info do you actually want? '74 mag 250 -'77 are pretty much the same engine ('74 head is different in the fin style), 1978 is a new (different engine), mainly cylinder & head design.
As for the 390's, 1977-1979 are the same, 1980 is a different engine ("big shaft") with different stud spacings etc. Not much interchanges with the earlier engines.
I can help with '74-'77 250 measurements & '77-'79 & 1980 390 measurements.
Let me know which you actually want?
K