Straightening forward or backwards is relatively straightforward, but straightening side ways can be difficult,except at the tips where the narrow timber is easier to manage. Once you get over 3/4 inch the timber starts to fight you and if you ask it to do too much you can get a split along the grain , so yes, get it thinned down, the heat will penetrate better and with less timber to fight you have more chance. If it were me, though, I'd be using dry heat