Stock - they had thick, conical washers on the nuts/bolts. That was to spread the load on the castings. I would use them. Your flanges look pretty thin. I'd really be worried about cracking those.
Yes, I am concerned also and was curious to the the load distributing characteristics of the conical washers from the 273 over a flat washer. I figured best to put those at the thin spots and possibly flat washers on the long bolts.