Use the stock maxx length for the front. I ordered UE's ti center drive kit and a longer ti dogbone for the rear. I used the UE cups so I can't tell you for certain that they are the same size as the HPI's.
i too used my existing UE ti centers, w/UE drive cups. i just measured the one that was in the back (exact same length as front) and it's 70mm. but as procharged mentioned, they may work in the HB cups (i have no experience with them).
if it doesn't fit, let me know and i can sell you my extra 70mm ti dogbone.