You might take a look at the "Marshall Tree Saw".  It is one crazy beast of a saw.  There is (or at least there was) one in the area a couple years ago that was claimed to have cut   +36"  trees with little effort other than moving the skidsteer around the tree to make different cuts to get through the entire diameter.  It had been used on over 200 acres by the owner and he had purchased it used, -- he had very good things to say about it.   

I like the design of it, it's a sheer/saw combination.  The teeth 'grab & cut' at the same time so the sheer action doesn't slide the skidsteer away from the tree. 

http://www.marshalltreesaw.com/SkidSteerAttachments.asp

It is a fairly heavy unit though so it might be a bit much for a 773, don't know for sure.

fwiw, I've got a hyd post hole digger from Harleman Mfg. and I really like it -- he hadn't started building saws yet when I bought the phd and I haven't seen any of them except in ads.  Nice outfit to deal with though.