If it where I, i would use the dsi for guiding and get a decent cooled CCD! you will never miss the target on that one! and the guider is fine for a 1200mm focal length, so 1800 shouldn't be much different! the SW finder is a f4.
gary, i love where you mounted your finderguider!
might have to look into that too.
Currently i have mine set in the original skywatcher finder foot and have had no issues what so ever even up to 20 min guided images! i have still to do a bit more testing with the 10" newt but guiding in maximdl i have a guide error <.5 pixels! which is brilliant in my mind!