Chop it!! Lee.....
Seriously its all a compromise between tube diameter, focuser height, secondary size and how far the tube extends past the secondary etc..
With a few quick calculations your 50mm is too small, if you go up to 63mm which Eric has in his that should give you a much larger field of illumination (about 20mm, with some guess work with your dimensions) at 100% illumination which would not be too bad.
If it was mine and you are going to do a bit of imaging I would just cut it to start and take some long exposures through it to see how bad the vignetting is. Then go from there..
With mine I have a 55mm secondary in my 6" and a low profile focuser which is 45mm high. 100% illumination is nearly 25mm.
Go on Chop the dinosour...
Cheers Gary