Why RTV and not the O-Ring?

Yes! Just flush.
 

1pc head gasket + RTV only.

Back in the 80's I had the Infamous "PUPPET" Of RaceVisions, the chassis builder. He aluminum heli-arc the groove. Then I would surface the block. Looked like the original 73 Z1 blocks
He actually taught me how to weld aluminum. WE used to say Pup could weld aluminum to toilet paper! He was the best. I started and practiced on a stock block with big liners. When big blocks 1st came out. That stuff was expensive! I just started to fill aluminum in along all the fins. Just kept going, ugly as hell!
After  a few months of playing around, we milled the sides flush. I used that block on an old 1260 turbo, alky engine from Broadway Kawasaki, when Richie Rich Racing was around before the FBI busted them with a few Popular named racers in 1984

I have always used blue HYLOMAR gasket spray to seal all my gaskets also.

+1 on the Hylomar. I’ve never had anything leak with the stuff.
I have a twenty year old tube that’s nearly all squeezed out and the Hylomar HPF still left is as good as the day I bought it. The new Hylomar hazes over and the tube goes hard in a couple of years however.

Heres the good stuff but it can’t be found anywhere. You can buy Hylomar but not Hylomar HPF.

But why designed mama Kawasaki later on a two piece gasket with One ring