IMO i would definitely NOT purchase a step down ring period using the FF format. Julian will give you guys sound advice on this subject so shoot him a pm if he dont respond soon to your inquiries.
I had considered on getting the below but as of yet have not since i have received variable opinions from several different folks:
Seems to me if your shooting landscapes at f8.0 the above 82mm CPL`s should work fine without any vignetting even on the 5D MKII but IMO its best to ask direct users first and visually see some examples.
It's mainly because I have a 77mm ND .8 that I want to use for film (also have a 77 cpl). I laid the filter on the lens last night (camera was sitting on it's back so the filter wouldn't move) and I shot directly at a light in my apartment. From the best that I could tell, there was no vignetting. I'll probably pull the trigger on a $15 step-up but I'll report back with results.