a better bet is still 77mm.. ND110 is rather expensive filter and you wud not want to buy one for 67mm and then, 1 for 77mm...get a 77mm...and find a 67-77mm step up?
ND110 has been out of stock for a while now. You have do a special order to reserve one. You can do it with one of the people selling in mass sales, or one of the stores selling then.
Or you can try Hoya ND400, which is around 8 stops and is more neutral in color than the B+W ND110.
firstly u must know what's the filter size of ur lens...
for each lens there's a fixed filter size, so assuming ur lens is 67mm, it'd only fit 67mm filter directly, unless u use a step-up, step-down filter adapter