I used to race a 1296cc MkIII many years ago. Spits were one of the best handling production sports cars if you did a few simple mods. We used to de-arch the rear leaf spring simply by flipping over one of the leafs which then gave a bit of negative camber or you could back then buy a competition leaf spring already de-arched. Some people then added a camber compensator, but many felt it wasn't needed once the rear leaf spring was de-arched. I took mine off and ran without it with no ill effects, but definately run a camber compensator if you are using a stock set up. We then added comp front springs, a fatter front sway bar and Konis all around. My old Spit was pretty fast in the day. It quite often won its class in regional races back in the 1970's. You can get decent power with the right engine mods. A 1500 with twin webers/cam/headers/CR is nice.
Google for the Spitfire forum. I know you'll get lots of info there..