heating the chip itself with hot air set to 350 C results in a consistent temp of 180 C.  So the chip is absorbing 170 C.
So if the solder has a melting point of 158C, I should be good.  There is a 20 degree difference between top heat and bottom heat, which is going to have to be acceptable.  I may start off with a lower hot air temp, maybe 320 C.  That should result in the top heat being 160C and the bottom heat right about 160C.  If I don't get good results, I will have to step up the hot air.  
I'm not sure if I want the hot air to be 30 degrees hotter than the board temp.  I think I would rather have both temps be as close as possible.  I could just heat for a longer period of time.  That way the heat distribution should be pretty even.