I sorta get the feeling that the convex washers are going under the small tray screws. If that's right then the washers go on like upside down saucers; the smaller contact side against the screw-heads. They always go on that way no matter the application.
Washers for main bolts or headbolts should be flat and hardened. The factory main bolts AFAIK, do not have washers, at least I have never seen washers on any engines I have torn down, including 340s.
If you flip the stockers over and look under the heads, you will find a raised circular area there, that prevents the hex-corners from digging in.
If you have received new mainbolts with washers, they will need to be correspondingly longer to maintain the thread-engagement in the block, like of the stockers.
Hardened washers are usually bluish in color, and will resist being scratched by a small file. I install them, smooth side under the head, except on aluminum; then smooth next to the aluminum, unless the bolts have the raised circular areas, then smooth under the heads .
Washers are usually stamped, so the side the punch comes down on will have downward curved edges; this is the smooth side. The underside will have a sharp perimeter that digs into aluminum. I usually rub it off on Emery paper; then it's smooth side under the head again, filed side against the aluminum.
