It sounds like you haven't made tamales before, either? I discussed this on the thread titled GE roaster oven 18 quart, 11/06. But here is further info with an additional choice for temps.

About a 1/2 inch of water is put in the bottom of the oven, then a rack. The tamales are stood up on their folded edge, leaning toward the center, and cooked with the steam- you can put a heat-proof mug in the middle to help stand them up, if you want. Cook one hour at 375 degrees, or 2 hours at 300-325, replenishing water or broth with hot water or broth to keep the level/steam up. It depends somewhat on the size you have made.

If you pour the sauce over instead of using steam, it takes longer, the sauce should come about 1/2 way up the side of the tamales.