Im not sure what your Boiler looks like or the material of the door in question but a few tips and pic of a typical easy glass install.
If your burning coal you should be using Ceramic NOT Tempered glass.
Photo shows simple straps/brackets that hold the ceramic in place (be VERY careful not to put to much pressure against the ceramic but just enough to put a little pressure against the glass gasketing)
In most cases cutting a hole in the door should not be a problem but bear in mind you don't want to loose structural integrity of the door (warping over time). If the door has any built in gussets in the casting or steel they are probably their for a reason and you should try to place a window in between these.
Also be aware that you don't need a large window to achieve your desire of being able to view the fire (a 4x4 would serve just as well as an 8x8 and might make your job a lot easier and cheaper).
post us up some pics when your done so we can see and to give a finish to the thread