Yeah, I'd recommend going with the EH72FI. It wasn't a difficult swap, and even though you have a slightly different setup you should be able to use the pics in my thread as a general reference.
http://www.tractorbynet.com/forums/power-trac/202389-pt422-engine-swap-completed-pics.html
The photos and description Vacuous_One posted in the thread Scott linked up should be more like what you will be experiencing. If shaft size is different for the coupling, contact Terry at PT. I think the newer models use the bigger shaft now and they have the proper couplers available. The only other difference is the extra fuel line back to the tank, and that the motor might get a bit hotter than the EH65. Otherwise the effort should be similar to swapping in the same model. I've had some issues with heat soak of the fuel line when mowing for extended periods since the upgrade. I have an extra fan turning on with the hydro cooler fan switch, but still lift the hood when shutting off and letting it sit... that seems to keep the engine starting up easily once I get back in the seat. You can still use the stock exhaust too. Check my thread for the company I purchased the engine from (equipatron.com). I had a good experience with them and the total price was $1880 with the shaft size upgrade (1 7/16") and shipping.
Good luck!