20" is 1974-79, A,B,C and E, 318 thru 440
3462190
18" is mid/late 70's up, could be hd cooling on a slant. parted a bunch of those.
3870107
they both have a healthy pitch to them.
I'm running the 18" on my Demon.
2 core alum rad, JSD G126, standard pump, no shroud(fitted one but doesnt seem to...