What have I got here? Magnum 5.9 Flex plate identification

The total diameter means nothing as long as it fits in the bellhousing without hitting anything while rotating and has the correct diameter bolt pattern for your converter (10 or 11"). The original flexplate you got with it might work with a 904. By the looks of it it has at least 2 sets of holes for different diameter torque converters. My guess is the inner set are the right size for a 904 converter.