These engines were originally built with tri-metal bearings. Tri-metal is a Steel shell, covered with a lead copper layer and the upper surface is a thin coating of Babbit (lead-tin-antimony) overlay. The babbit and copper lead will embed dirt and keep it away from the shaft and conform (to some extent) to any irregularities in the journal.
Bi-metal bearing is a steel shell with a hard aluminum layer. Bi-metal bearings don't tolerate irregularities in the journals, and don't tolerate dirt and debris well at all.
My personal thoughts: I have always used Tri-metal bearings over Bi-metals for these engines. In fact, the old Vandervell bearing had even a softer upper layer of Lead Indium that is/was preferred by many Buick engine builders. The Vandy bearing are almost all gone now. But there are other good choices still available.
I have a good article about bearings characteristics that I'll post later.
