Most popular tach in the 70's and other stuff

I agree the Sun Super will look period correct. I couldn't afford one back then so I had a RAC Gas Rhonda signature tach in my '69 340 Swinger back then.
For your other questions, I never had - and don't remember any of my friends having - an AM/FM converter back then. FM stations back then didn't play popular music, they played "elevator" music that our parents listened to. All the good rock & roll SoCal stations were AM. It would have to be an underdash 8-track to look right if you're going for period correct.
Stickers? No such thing back then. They were all waterslide decals - try and find any of those today. But as far as subject matter? I had Keith Black Racing Engines, NHRA Div. 7, and a Holley decal on each of my rear quarter windows.