I look at it as 3 sz heads. I have a '69 - the "M" head good for the 2v direct mount
we have some good examples of how NOT to do it in one or 2 threads (that's pretty easy)
a small log, hex and lrg log. '77 + is lrg but 80/3 is the right 1 to source. Jegs comes in and out of AKT (or some such co) but 300$ is a bit much & not sure which log (see their 'yr application' 4 details).
I like the national junk yrds. One problem - a guy in Lakeland Fla sold me a '75 for '77 + & I could not correct the prob. He thinks he knows how to read castings & can't (wuz it AAA junk yrd?).