yes I just torch the old rubber bushing out and made a solid aluminum bushing. the stock rubber bushing is terrible when it comes to putting a lot of load on it when turning left or right. swing arm likes to move all over. especially when using a larger rim(10" or12")
I think with the type of driving you will be doing I would keep the front suspension the stock height and spring and the rear after extending the arm about 5" long. something else to keep in mine too. when making the arm longer you might want to cut and lengthen in a way so you can maybe also modify it so you can install a jackshaft in the swing arm to get the right amount of gearing. you might not get it with just the stock factory way? just don't know off hand the kind of road course you are riding on?
back to your swing arm bushing. you might want to make a short aluminum bushing and have a good sealed bearing on each end. you might have a lot of movement up and down. for me I just drag race the bike for the most part and really don't se a lot of movement. make sure you have a good rounded knobby tire so it can roll easy into the corners and hold the bike up. also I have notes in the past with the stock length rear arm the bike likes to buck you off when going into a corner fast when you try to lay it down when turning. so longer may help the problem go away and don't forget the motorcross style handlebars too...…..