Well you will need to measure in that case. However, I would guess going with .450" would be best. My concern with .400" would be that the front hole would land too close to the existing drilled pin groove in the barrel.
The rear screw hole will still be opposite the gas port. SLR sets the rear screw and gas port spacing such that the block fills the space that would originally be occupied by the handguard retainer cap.
Sent from my iPhone using Tapatalk