After looking over various case dimensions I settled on Jamison/Captech .40-70 Sharps Straight brass to form .375 Flanged cases from. A dab of Imperial Sizing Wax and they slip right into the CH-4D FL sizing die with scarcely any effort.
I will work with the Hornady 270-grain .375 RN in working up loads, once I get my Lee Metford from Von Gruff and can have the barrel rebored and reamed.