I'm not sold on those. I know lots of people who put belly mowers on obsolete Allis's or Deeres, but a fergie is too ??? useful? versatile? To tie up with a belly mower. Unless you have no other jobs for it-ever. AND you won't really manuvuer? as well as with a 3 point, you will leave heaps more 'holidays' in corners you will need to mow by hand.
I agree your 3 point 'slasher' is not what you need on a lawn at all, but these 3 blade close quarter jobs- this one is an older Ford 920? doesn't even need a live pto, or even a override, there is no momentum in there. Light enough to manhandle it. Look for one of these and Bob's yer uncle mate!.