How would your expert know the compression before you have heads? Are the pistons flat top, domed, above deck?

Maybe you misinterpreted the engine builder. Did he say your motor: Will support 400-450 HP

A 400-450 HP will not have the same idle and street manners as a stone stock ~300 HP motor.

Intake: edelbrock air gap or RPM

Heads: stock ported or edelbrock

Cam: around comp XE 284 spec range. “Rough idle”

Adjustable rockers to support that cam specs

Demon/Quick Fuel/etc 750 vacuum secondary

TTI or Dougs step headers