Pump gas compression

I answered in the post before in the Race section - expanding on that it's the combination of parts that makes the engine prone or less prone to detonate. Looking at this build, I would not want it under 10:1.
IMO it's giving up a lot and the bigger engines need the compression. 8% is the difference between a 500hp and a 540hp engine and it will affect torque to a greater extent. That 40hp will affect ET and IMO there's no down side so long as you can get somebody's 91 octane with 10% ethanol out of the ground.
If you were having me build it I'd go with that 4.125 stroke you want, the shortest rod that clears the counterweights, and a 20cc dished piston coming proud of the deck .005". I don't know if the 5.7 Chevy rod will clear with the smaller Chevy piston pin size but I'll always trade piston height for rod length in a race engine.
All that being said - conservative is always the safer course - your builder knows that.