Eric
Brass is not a good bearing material, if you intend running the loco do not use it, gunmetal is a far better material and as Fizzy points out you can use cast iron.
Aeronut, If you have made the axle boxes stick with the bronze bearings, if not you can have the choice. I would say to both of you make sure you can get adequate lubrication into the axle boxes, over the years i have seen several loco drawings where the provision of oil was poor or even non existant, it is better to have slightly to much than not enough oil.