Cast iron versus aluminum was not a date change but a series change. 40 series H42 and H42 were always cast iron. 60 series and later 70 series were always aluminum. Four speed aluminum covers the shift tower was further back on aluminum covers. All H41 and H42 four speed transmission the tops...
Would be really hard to comment on what causing the problem without actually driving it in person. All the shift forks would have to be in the correct collars for it shift all the gears including reverse. The H42 is not like old GM three speed columns shifts from the fifties and sixties I drove...
Kept my three speed clutch back in 1994 when I switched to a H42. Works fine as long as all the parts stay the same, including the flywheel. The clutch style didn't change until 8/74. Same time the fork, throw out bearing and flywheelchanged.