Viscous fan package?

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 need it)
zero deck 60+ 360, KB 107's, 750dp, airgap, big valves, ported heads, comp xs262(?) cam, 727 w 3400 stall, 4.10 gear and dougs headers.
Runs about 180-190. No heat issues even on a 110* day.
Of course it will creep up sitting at a light but neutral and a couple hundred extra rpms and its good.
Shroud will help reduce low speed/traffic heating.