For what it's worth...when I built my 605 the kit came with diamond pistons. At the time I was using an Indy 426sr head,(stock size valves with stock valve angles). I had a Bullet custom roller ground up for it. When mocking things up I found I didn't have enough p/v clearance and had the valve reliefs cut. After buying a set of Indy Hi-Po heads with the big valves and exhaust tilted 1.5 degrees I had loads of p/v clearance. I'm guessing the pistons were aready set up for the altered valve angles.
You will also have a bit more exhaust lift with the tilted exhaust valve as it moves it closer to the center of the rocker arm.