Why do you want them to vomit?
They will have lots of room to vomit...... Our rule of thumb for haze layer is half ground temperature x1000, so generally its in the low 20's, so for the majority of the time, you will be ok above this, obviously there are days when its great to climb straight to 410....
As for the older 737, I would attempt to add 1-2 internal tanks to increase range.
Mutt