Drool tube heads run from '60-'74. The changeover may have been in the '74 model year.
Reality is if you are going to rebuild one anyway there is negligible difference between the vintages. The only real drawback to the '75-'80 heads is that you can not pull the lifters out with the head on. Not a big deal to me, but some people stress about it. On my junk if I have to pull the lifters it's usually because there is a major issue anyway.
You can also use the '81 and up hydraulic head on any block, but you do need the correct valve cover for it. It is different and an '80-down one will not fit.
