Hi Greg,
Handholding a graduated neutral density filter in front of that particular lens will be problematic. I tend to hold the filter flush up against the lens. With the aspherical element, unless your filter is orthogonal to the lens and parallel to the image axis, you will introduce surface reflections of your surrounding environment. I feel that it would be prudent to invest in a filter holder for such a beast.
Get the Lee filters, straight off eBay. They do the whole 1-, 2-, 3-stops and beyond. The soft step filters fade gently from halfway up the filter to the top of the filter, whereas the hard step filters typically gradate from nothing to maximum in the space of 5% of the filter length; on an 6x4" filter, that equates to 7.5 mm.
The hard step filters are typically used for flat horizon/ocean shots where there is no subject or object that could be obscured, whereas soft step filters are used primarily for scenes involving subjects such as mountains, and, the like. Reverse filters reach maximum density about 2/3 to 3/4 through the filter and then soften out again. This is so that cloud features don't turn too dark at the top of your frame. I avoid this typically by gently blending in a second exposure or a pushed double-processed RAW.
You always have the ability to double stack filters, too. Typically, beyond 3-stops of neutral density, the filters shoot up in price quite dramatically, like, 50%+ hikes. So, you can get a couple of cheaper ones and stack them together to get the density required.
A recent 3-stop hard step filter in action:
http://users.tpg.com.au/octane2/b.html
Hope that helped.
H