This is the best possible situation because now you can test the two housings for yourself. I would do as you originally planed and run the Precision first because you will learn the difference between the spool times of the smaller and bigger engines. Then change to the Garrett and learn the difference between the two turbine housings which will make you the go to guy when someone asks the same question you asked when you started this thread. You won't be guessing,you'll be sharing your real world experience. You will be able to share your experience about two different sized engines and two different sized housings. If you change to the Garrett before running the new engine,you won't be able to tell anyone how much the bigger engine helped nor how much the smaller housing helped.I was just going to run it with the Precision .63 and see how I liked it and go from there but as fate would have it a Garrett .63 landed in my hands after I started this thread