A2 and A4 carry handle sight guts compatibility, and .170 FCG question

[masakari]

So, I'm building an A2 "clone", except that I'm making it how I think it should have been. That means lightweight barrel and A1 length CS stock.

Anyway, one of the things I'm considering is using A4 sight guts (6/3 0-600 yards) instead of the optimistic A2 (8/3 0-800 yards) sights.
I currently have a Colt A2 upper, and will be getting a Colt A4 carry handle.
Will I be able to swap the sights themselves between the two? Are they compatible?

Also, my lower happens to be an old Colt .170" FCG large pin lower, and the hammer is notched. Does anyone still sell a rounded (not notched) .170" Hammer?

Thanks guys