All good ideas.
Often you can just leave the bearing in the little bag they are shipped in to keep the ice off - just cut it off at the last minute.
As a guide heat the cases until they sizzle spit (or water if you must) licked onto the end of you finger. this means the cases have reached 100 degrees C +, which as we all know is the temperature water begins to boil (at sea level at one atmosphere of pressure etc etc).
Should you wish to install items such as valve guides or valve seats the casting usually have to be far hotter to overcome the size differential due to required interference fit (shrink fit). The best guide I have always used is to flick a finger load of water onto the casting and the water should boil as little angry spheres dancing around on the job. When you have to satisfy the requirements for a five thou shrink fit you can't muck it up and the item has to be bloody hot but don't overdo it!
Another tip to ensure some matrimonial harmony is to put foil or an old tray under the job to catch the oil and thus stop the whole oven smelling like the floor of your workshop!!!
As always I hope this helps.
Dave Mac