What is the lift capacity of your FEL and do you have a quick attach mechanism for it? Unless Markham has changed, I think they only offer a "universal skid steer type" QA or the John Deere style QA. If you don't have one of these, you would have to buy an adapter, such as a API Tach-All, which are pretty costly ($700) assuming they make one for your specific FEL. Or I guess you could weld some type of adapter if you have that ability.
Generally, for a 40 hp tractor, I would get a 48" light duty, which is still plenty stong enough, which cost about $950. Don't forget you will also need hydraulic controls and this could cost from several hundred up to $600 for the WRLong electric over hydraulic valve setup. You have several choices in how to set up the hydraulic controls.
I bought a heavier duty version of this grapple for my 70 hp Kubota ($1460), an API Tach-All QA ($700) and the WRLong valve ($600). My lift capacity is about 2500 lbs. The grapple weighs about 400 or 500 lbs. I think the light duty is about 380ish.
Costly! But it is a killer implement, by far my favorite for land clearing, brush removal, uprooting trees, and general picking stuff up and carrying it somewhere else. It is well designed, well built, and worth every penny. I just wish my FEL was skid steer QA, so I would not have had to buy the Tach-All. Hope this info helps. If you have other questions, please respond.