Modifying the cat may be more expensive than just finding another Cat set up for the park but if you are determined to modify your existing machine which I understand. You first will have to extend the mount so that when you do add a longer rams you don't pinch your cab. It will have to be heavily welded since you will be basically extending the whole blade and mounts from of the machine. It shouldn't throw your tiller wight off but you may have to consider the overall balance of the machine depending on how much weight you add to the front.
what you will need:
-longer rams you might luck out and be able to find them through northern tool or similar company
-extended front mount
remount the rams and hoses so they do not get pinched when you add the new range of motion.
The welds are the key and if done even slightly below perfect you will probably rib the blade off once you start pushing a lot of snow.
Best of luck. Sounds like a fun project. The 275 are nice to operate in the park.