Heads, what are these ?

That casting was used from '70 through '72 on 340 4bbl, 6bbl. and 360 engines. The machining was different depending on actual application. The letter cast into the head after the number indicates early and late castings. A would be first year, G was much later. Not sure the exact breakdown of letter to year/month, but, it gives you an idea. I don't remember seeing a letter higher than a G, so, I would guess it's a late '72 casting.