The DR650 is well known for having crap header welds where the flange that bolts to the head is welded to the header pipe. On my DR650 I took off about 3mm all the way round and that still left 2mm in place.
Today I had to replace the Cam Chain Tensioner gasket on Jennifer’s DR250 Djebel, (Which is for sale) so I pulled the header off the 250 and found the 250 is exactly the same as the DR650.
I managed to take the lumps on each side out but ran out of grinding discs for my dremel.
So if the DR 250 is like this then I imagine the DRZ 400 is most likely to as well.