I use a wheel-weight/tin alloy. I heat at 450 for an hour then drop in water.
Increasing the temp will increase hardness, but you can deform the bullets. You could find the max temp for your alloy by gradually increasing the oven temp every 15 minutes until the bullets start to slump. I've never bothered to find out the max temperature for my alloy and my bullets are plenty hard enough for maximum .44mag loads.