Mark,
have you seen Reiecke's design page?
http://www.stsci.de/fg/pdf/reinecke.pdf
He uses an OAG body between the scope and the lens..... and an illuminated needle as a reference slit
That's the correct grating; I'd recommend starting with the 600lpm and then later ( or if you can afford it get both!) the 1200lpm ( # 3-3150)
This will give full spectra range coverage with the Canon.
I'm finding another issue I haven't seen noted elsewhere with the Littrow; I seem to be losing some contrast, which I think may be due to primary reflections from the surface of the collimator lens nearest the camera. It's a coated lens but with a pretty flat surface???!!