It really depends on what you want to use it for !!! If street duties and ocasional drag race I personally think you wont have to go any bigger than the 400hp GT25 these turbos can make well over 400hp if drag race only you could go bigger again but would be a bit of a dog to drive. For a daily driver the 350hp ray hall special trim GT25 would be optimum with great all round performance you could expect 200rwkw from this turbo with the rigth setup and still drive it around town all day without being a pig.
Good luck and let me know how it goes another freind of mine just got his rebuilt CA18 going last week it only has the 300hp turbo he is running it in at the moment will be intersting to see what it produces when run in then tuned for lots of boost. its in a little 1200 ute
heers Andrew