When you go up in size on either of the pulleys you have to get a longer belt, otherwise it will be waaaay to tight on the adjustment if you are even able to get it on the rear pulley in the first place. Yes, changing the belt can be a pain because you have to remove the swing arm, but it's worth it to get this mod done right. I don't play around with things involving powertrain. It always ends badly.
You normally can find a used belt/pulley from a wrecked bike or someone that's gone and change their final drive gearing for a reasonable price. I spend 150.00 on my pulley and belt off of a stratoliner and sold my gently used belt for 100 bucks and still have my stock pulley in the garage.. So.. um, I was out 50 bucks for the mod so far? Not shabby.
Drilling out the holes on the HD pulley shouldn't be a biggie. They are plentiful, so if you mess one up, it's not a big deal to go buy another. This would be a pretty easy do it urself mod.
When you are doing this mod however, MAKE SURE TO USE RED LOCKTITE WHEN INSTALLING THE NEW PULLEY ON THE OLD HUB! Very important. Sorry to yell. I thought that the locknuts were good enough but I didn't realize that the studs where not actually one peice with the hub. The studs screw into the hub itself and are independent. So, you have to locktite both sides. It's super important. I noticed mine was starting to get loose and couldn't figure out wtf was going on. I had to take it all back apart and redo it. My mistake is ur gain on this one. I was lucky and it didn't cause any damage because I caught it early.