check next to the distributer hole for the date.it will be large and say 75, or whatever year. i briefly checked wallace racing site and found no yh code in 74. 1975 yh was a 400. other years; 70, 72, yh code was 455. the y in yh tells you it was attached to an auto trans. check the code over the two center exhaust ports; it will say 6x or something, large numbers; then we'll know what kind of power it has.
check out
www.wallaceracing.com all of your engine codes can be decoded there.