I current have the windham upper you are talking about. 7.62x39 or ak round. The upper uses carbine length hand guards with the delta ring retention. You can replace them with an after market quad rail made to fit the delta rind end cap set up. I have a utg made quad rail that works well. I use it with a light mounted for hog hunting.

Typically, they are not interchangeable. The AR10 rifles' (a.k.a. 7.62x39mm AR rifles) barrel nut is a different size along with the rail height being different. If you are looking for a free float rail for that rifle, I suspect you need to look for an AR10 rail. As always, please consult with the manufacturer's documentation or contact the manufacturer directly.