I'm not sure a 3PTH tiller can be driven off of a PT-425. Tillers take a lot of power. The PT-425 PTO puts out slightly less than 12 horsepower, from which you have to subtract hose and motor losses. You would have maybe 10 net HP at the tiller's gearbox input, perhaps less.
The Power Trac tiller for the PT-425 is a relatively lightweight direct drive design, which avoids the right angle gearbox and chain/gears jackshaft to rotor losses of the mechanical 3PTH tillers.
For a PT-425 sized 3PTH tiller, it would cost probably $300 to $400 dollars to equip it with a hydraulic motor and hoses. Then you have to fabricate and install a QA plate and mount. Add those exenses to the cost of a used tiller, an item that doesn't come cheap, and you have an uncertain combination in used condition that may or may not work, but approaches the cost of a new Power Trac tiller.
Not a good idea in my book.