I'm no imager, but quite often the mirror in a newtonian needs to be moved forward to achieve focus with a DSLR. This is because of the position of the sensor. This can sometimes be achieved with the mirror adjustment screws, without drilling holes in the tube. At times the focusser needs to be replaced with a low-profile one. The thread below discusses the problem:
http://www.iceinspace.com.au/forum/s...ocus+newtonian