Flail mower, Alamo, changing from fine to coarse, How?

I recently picked up a alamo 74in 3 point hitch flail mower at auction. When I got it home, I realized it appears to have the fine cut cutting drum and blades (from looking at the info from flail master). I need to use it mainly for de-brushng, and mulching smaller dead fall. I am Mounting it on a JD 4066. I also have some rocks. My assumption is that I need the extra duty coarse cutting blades to do what I want. Flailmaster shows a different cutter shaft for coarse cutting, but it is rather pricey. What other options do I have? thanks, robert