Got a 383, need advice from my FABO brothers

The 915 heads with the 1.74 valves have the same casting number as the 915 heads with the 1.60 valves- the part numbers are different: 2806762 and 2806019.
So are the part numbers on the heads?
Otherwise I am not sure I would be able to tell the difference, or the size of the valves.
He also asked if I wanted open chamber or closed chamber, but from what I can find both the 915s and 516s are closed- and he suggested I take the open.
What am I missing here?
He knows his stuff, so if something sounds wrong it is probably me not understanding- but the part numbers/casting numbers above I looked up.

FYI: that pic of that orange haired whatever is freaking me out.