Would it be ideal to do a pericardial window when one has a mod-lg pericardial effusion along w/ left shoulder/blade, arm, neck, chest, & back pain.

Pericardial effusion: Most importantly the cause of effusion needs determination; a window procedure offer symptom relief and opportunity to secure fluid and tissue for lab analysis as a diagnostic exercise as well as relief of symptoms

Yes, esp. If echo +: If there is right ventricular compression by echo cardio gram, then sooner, the better. Distention of pericardial sac can cause symptoms you described.