David, That casting number when searched through
GMPartswiki comes back with multiple part numbers and applications for both car and truck. A 1970 Chevy Master Parts Book indicates that casting number under part number GM# 3789730 was for 1955-62 Pass cars w/11" clutch (6 cyl.). An earlier 1960 Truck Parts Book indicates it's listed for 1960 C10-30 - 6 cyl. (10" Clutch) as GM# 3705409 with the same casting number. Other books show that casting number for 9-1/2", 10" or 11" clutches in different applications. Just my $.02 worth, but take your flywheel and measure across the clutch disc mating surface as shown in an edited pic of yours below. That can be your guide to which clutch disc kit to use.
BTW - that "B1855" is the casting date code which would indicate February 18, 1955.
