You may find that you need two stepdown rings ...I have 72>55.
Thinking about the Lee holder and its expense... if one knows, see below. the size of the filter I wonder if it would fit in one of the numerous makes of holder which are often found at auction sites at quite reasonable prices. Normally woth these holders there are three parts. The holder for the filter, the filter, and an adaptor ring behind the holder. The holder comes is three sizes "A" "P" and 100mmx100mm .... the Lee 'Big Stopper' is 100x100. and priced at UKP 104.95 ... the holder is UKP59. To read up on them I suggest you go to
www.SRB-Griturn.com and work you way through their catalog. I have dealt with them and have a colored hard copy from my last purchase. Prices quoted from it and I am not sure how old it is
The adaptor ring is UKP20 for the standard version and UKP40 for the Wide -angle lens version.
There is also I see the 10 stop Hi-tech version for UKP72 in the 100x100 size or UKP48 in the 'P' size [85mm]
SRB also have their own line of filters but when I stacked three of them I got a bad cast, pink
and make A and P holders but not the 100x100.
With a 55 lens thread you can use the smallest holder but it looks as if you will need a P holder for the Big stopper or equivalent.
Since you need to remove your lens hood to mount the holder you may need to think about a lens hood. The holder has three or four slots and you can get, or make as I did, a lenshood with a slide to suit the holder. [UKP 105<209 dependiong on model]