I hate to say this but a dealer is your best bet! You should get the one built for the tractor and it should give a good level of performance. I only recommend leaving them on for about an hour max so you don't run up your electricty bills.
Get the Kubota one, it is about $40, and threads right into the side of the engine block. That's what I did for my B7610. The whole job was done in under an hour, including the coolant change, which it was do for anyway.