If you use alcohol injection and/or race gas you could just get by with 50's, if you plan to max out on 93 octane without alky then 65's are the minimum. I'd either go with the 65's (also called 60's) or if you plan to step up to a bigger turbo later go ahead and get your ecm modified and go to 72's now instead of buying twice. I ran 72's and a 54 for several years - didn't need all the 72's could do but shouldn't ever have to buy bigger.