On a John Deere, the topmost hole (for the top-link) is for the heaviest loads...that's the one I would use (my backhoe uses two plates as a subframe). But, you really want to look in your TC-40 operator's manual or call your dealer. As IH3444 wrote, 3PH backhoes can overstress the tractor's frame (which isn't really a frame...the engine and transaxle spreads the stresses...but the connection between the engine and transaxle bears the brunt.
There have been a number of posts about TC-30s breaking.
As Btown wrote, a lot of folks have used 3PH backhoes with no problems...just realize these are not dedicated, purpose built backhoes. Your TC40 is a compact tractor that can run a backhoe for fairly light duty digging.
One thing I would avoid, even with my subframe mounted backhoe...you don't need to raise your rear tires off the ground. The stabilizers are just that...to stabilize the tractor when digging and, especially, when swinging the boom.
Here's an old TBN thread on the subject:
http://www.tractorbynet.com/forums/kioti-owning-operating/253017-backhoe-question.html