I think you'll be pleasantly surprised with the lightness of the phantom to put a 68/45 on it. It'll work just fine though, I really like 45/45's on phantoms myself. For the bottom line, get an ASA with a wedge if you haven't yet. Feels a lot nicer.
Yes, a flathead screwdriver will work fine. I just took my barrel off, tweaked it, rinse and repeat.