Martin another point may be the spacing between your lens and the camera. I am sure the Canon engineers designed the lens to be a certain distance away from the sensor. Perhaps you should measure that and make sure your lens is in that zone.
Also I think the Pentax 67 lenses are good bang for buck here. They have outstanding backfocus at around 89mm (among the highest of any camera lens).
I use FLI PDF focuser, FLI CFW and Proline and an adapter made by Precise Parts that fits on the PDF.
I am using Pentax 67 55mm F4, 165mm F2.8 and 300mm F4.
All are non-ED and relatively cheap and plentiful on ebay.
The 55mm I use at F5.6 and 2x2 binning. The 165mm can be done wide open at F2.8 and on a demanding Proline 16803 chip is sharp corner to corner. The 300mm also is sharp.
Examples of the 55mm at F5.6 and the Proline 16803 are here:
http://www.pbase.com/gregbradley/image/140786051
http://www.pbase.com/gregbradley/image/140793803
There is some chromatic aberration that shows up as a bit of magenta around brighter stars but its easily fixed in Photoshop.
Also at 2x2 the stars are a bit larger than at 1x1( needs a small amount of deconvolution) but I was getting awful seagull outer stars at 1x1 on the Proline. 2x2 though magically stopped that.
There is also a 45mm F4 Pentax 67. They go for a few hundred US dollars on ebay.
An 8300 chipped camera would be a piece of cake for these lenses.
Your Canon F1.2 is probably worth a lot and is best used on a 5D mark iii!
Greg.