Always fun. First be sure to remove the spark plug for safety.
You'll find it a tight fit and you've probably already figured out the blade belt has to come of the bottom of the engine stack pulley first. I'd drop the whole deck since it comes off pretty easy just to give myself the extra room. There's a single idler pulley that swings into the belt close to the transmission. If you remove the idler while your installing, you'll have a bit more slack to play with.
You'll need to roll the new belt onto the transmission pully and it'll be tight, but you should be able to work it in there. ONLY IF YOU REALLY HAVE TO, you can loosen the four shoulder bolts that hold the transaxle to the frame to gain a bit more clearance between the transmission and the frame. You'd have to jack the frame up while the wheels remained on the ground. But I've only seen that as necessary when the frame was tweaked, so keep that one in you back pocket and forget it for now.
Your belt guides can be fun too. They have flange nuts holding them from the underside of the frame and are really just long hex-head bolts that are coming down from above. You'll need to loosen them or "tweak" them out of the way most likely. When you're done, they should be back in position 1/8" from the belt when it's seated. Otherwise the belt may not disengage properly.
Also, save yourself some potential headache and check all the pulleys for wear as you go. You don't want to do all that work and have the belt shred on you. When pulleys wear, the shoulders get wider and allow the belt to ride deeper in the groove, often getting caught and damaged in the process. Also check the bearings on the idler for free rotation.
I hope this is helpful! Keep me posted.